So the greenpak software didn't want to start as I'm missing a newer version of glibc. Slackware 15 is currently running glibc 2.33. I could switch to current which runs 2.40 or just install the glibc package from the current?

There is probably some other dependencies missing so I checked.

bash-5.1$ ldd /usr/bin/GPLauncher
/usr/bin/GPLauncher: /lib64/libc.so.6: version `GLIBC_2.34' not found (required by /usr/bin/GPLauncher)
        linux-vdso.so.1 (0x00007fff6a735000)
        libQt6QuickControls2.so.6 => not found
        libQt6WebView.so.6 => not found
        libQt6Sql.so.6 => not found
        libQt6QuickWidgets.so.6 => not found
        libupdaterclient.so.1 => not found
        libcoregp.so.1 => not found
        libsettingsmodule.so.1 => not found
        libgui.so.1 => not found
        libQt6Quick.so.6 => not found
        libQt6Qml.so.6 => not found
        libQt6Widgets.so.6 => not found
        libQt6Gui.so.6 => not found
        libQt6Core.so.6 => not found
        libstdc++.so.6 => /usr/lib64/libstdc++.so.6 (0x00007feb22910000)
        libgcc_s.so.1 => /usr/lib64/libgcc_s.so.1 (0x00007feb228f5000)
        libc.so.6 => /lib64/libc.so.6 (0x00007feb22716000)
        libm.so.6 => /lib64/libm.so.6 (0x00007feb225ce000)
        /lib64/ld-linux-x86-64.so.2 (0x00007feb22b5e000)

QT6... Okay slackware current has QT6, I will install that package and try again. Think there will be missing dependencies for QT6 aswell.

bash-5.1$ ldd /usr/bin/GPLauncher
/usr/bin/GPLauncher: /lib64/libc.so.6: version `GLIBC_2.34' not found (required by /usr/bin/GPLauncher)
/usr/bin/GPLauncher: /lib64/libm.so.6: version `GLIBC_2.35' not found (required by /usr/lib64/libQt6Quick.so.6)
/usr/bin/GPLauncher: /lib64/libm.so.6: version `GLIBC_2.38' not found (required by /usr/lib64/libQt6Quick.so.6)
/usr/bin/GPLauncher: /lib64/libc.so.6: version `GLIBC_2.38' not found (required by /usr/lib64/libQt6Quick.so.6)
/usr/bin/GPLauncher: /usr/lib64/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by /usr/lib64/libQt6Quick.so.6)
/usr/bin/GPLauncher: /usr/lib64/libstdc++.so.6: version `GLIBCXX_3.4.32' not found (required by /usr/lib64/libQt6Qml.so.6)
/usr/bin/GPLauncher: /usr/lib64/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by /usr/lib64/libQt6Qml.so.6)
/usr/bin/GPLauncher: /lib64/libm.so.6: version `GLIBC_2.38' not found (required by /usr/lib64/libQt6Qml.so.6)
/usr/bin/GPLauncher: /lib64/libc.so.6: version `GLIBC_2.34' not found (required by /usr/lib64/libQt6Qml.so.6)
/usr/bin/GPLauncher: /usr/lib64/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by /usr/lib64/libQt6Widgets.so.6)
/usr/bin/GPLauncher: /lib64/libm.so.6: version `GLIBC_2.35' not found (required by /usr/lib64/libQt6Widgets.so.6)
/usr/bin/GPLauncher: /lib64/libm.so.6: version `GLIBC_2.38' not found (required by /usr/lib64/libQt6Gui.so.6)
/usr/bin/GPLauncher: /lib64/libm.so.6: version `GLIBC_2.35' not found (required by /usr/lib64/libQt6Gui.so.6)
/usr/bin/GPLauncher: /lib64/libc.so.6: version `GLIBC_2.34' not found (required by /usr/lib64/libQt6Gui.so.6)
/usr/bin/GPLauncher: /lib64/libc.so.6: version `GLIBC_2.38' not found (required by /usr/lib64/libQt6Gui.so.6)
/usr/bin/GPLauncher: /usr/lib64/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by /usr/lib64/libQt6Gui.so.6)
/usr/bin/GPLauncher: /usr/lib64/libgcc_s.so.1: version `GCC_12.0.0' not found (required by /usr/lib64/libQt6Gui.so.6)
/usr/bin/GPLauncher: /lib64/libc.so.6: version `GLIBC_2.34' not found (required by /usr/lib64/libQt6Core.so.6)
/usr/bin/GPLauncher: /usr/lib64/libgcc_s.so.1: version `GCC_12.0.0' not found (required by /usr/lib64/libQt6Core.so.6)
/usr/bin/GPLauncher: /lib64/libm.so.6: version `GLIBC_2.35' not found (required by /usr/lib64/libQt6Core.so.6)
/usr/bin/GPLauncher: /usr/lib64/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by /usr/lib64/libQt6Core.so.6)
/usr/bin/GPLauncher: /usr/lib64/libstdc++.so.6: version `CXXABI_1.3.15' not found (required by /usr/lib64/libQt6Core.so.6)
/usr/bin/GPLauncher: /usr/lib64/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by /usr/lib64/libQt6QuickTemplates2.so.6)
/usr/bin/GPLauncher: /usr/lib64/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by /usr/lib64/libQt6QmlMeta.so.6)
/usr/bin/GPLauncher: /lib64/libm.so.6: version `GLIBC_2.35' not found (required by /usr/lib64/libQt6OpenGL.so.6)
/usr/bin/GPLauncher: /usr/lib64/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by /usr/lib64/libQt6QmlModels.so.6)
/usr/bin/GPLauncher: /lib64/libc.so.6: version `GLIBC_2.38' not found (required by /usr/lib64/libQt6Network.so.6)
/usr/bin/GPLauncher: /lib64/libc.so.6: version `GLIBC_2.34' not found (required by /usr/lib64/libQt6Network.so.6)
/usr/bin/GPLauncher: /usr/lib64/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by /usr/lib64/libQt6Network.so.6)
        linux-vdso.so.1 (0x00007ffdc30ab000)
        libQt6QuickControls2.so.6 => /usr/lib64/libQt6QuickControls2.so.6 (0x00007f44d29d1000)
        libQt6WebView.so.6 => /usr/lib64/libQt6WebView.so.6 (0x00007f44d29be000)
        libQt6Sql.so.6 => /usr/lib64/libQt6Sql.so.6 (0x00007f44d2964000)
        libQt6QuickWidgets.so.6 => /usr/lib64/libQt6QuickWidgets.so.6 (0x00007f44d2943000)
        libupdaterclient.so.1 => not found
        libcoregp.so.1 => not found
        libsettingsmodule.so.1 => not found
        libgui.so.1 => not found
        libQt6Quick.so.6 => /usr/lib64/libQt6Quick.so.6 (0x00007f44d2161000)
        libQt6Qml.so.6 => /usr/lib64/libQt6Qml.so.6 (0x00007f44d1adb000)
        libQt6Widgets.so.6 => /usr/lib64/libQt6Widgets.so.6 (0x00007f44d12d6000)
        libQt6Gui.so.6 => /usr/lib64/libQt6Gui.so.6 (0x00007f44d08ab000)
        libQt6Core.so.6 => /usr/lib64/libQt6Core.so.6 (0x00007f44d01bf000)
        libstdc++.so.6 => /usr/lib64/libstdc++.so.6 (0x00007f44cffab000)
        libgcc_s.so.1 => /usr/lib64/libgcc_s.so.1 (0x00007f44cff90000)
        libc.so.6 => /lib64/libc.so.6 (0x00007f44cfdb1000)
        libxkbcommon.so.0 => /usr/lib64/libxkbcommon.so.0 (0x00007f44cfd6b000)
        libQt6QuickTemplates2.so.6 => /usr/lib64/libQt6QuickTemplates2.so.6 (0x00007f44cfac2000)
        libQt6QmlMeta.so.6 => /usr/lib64/libQt6QmlMeta.so.6 (0x00007f44cfa96000)
        libQt6QmlWorkerScript.so.6 => /usr/lib64/libQt6QmlWorkerScript.so.6 (0x00007f44cfa82000)
        libQt6OpenGL.so.6 => /usr/lib64/libQt6OpenGL.so.6 (0x00007f44cf9d5000)
        libGLX.so.0 => /usr/lib64/libGLX.so.0 (0x00007f44cf9a2000)
        libOpenGL.so.0 => /usr/lib64/libOpenGL.so.0 (0x00007f44cf974000)
        libQt6QmlModels.so.6 => /usr/lib64/libQt6QmlModels.so.6 (0x00007f44cf88c000)
        libQt6Network.so.6 => /usr/lib64/libQt6Network.so.6 (0x00007f44cf66f000)
        libm.so.6 => /lib64/libm.so.6 (0x00007f44cf527000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f44d2a23000)
        libEGL.so.1 => /usr/lib64/libEGL.so.1 (0x00007f44cf512000)
        libfontconfig.so.1 => /usr/lib64/libfontconfig.so.1 (0x00007f44cf4c9000)
        libX11.so.6 => /usr/lib64/libX11.so.6 (0x00007f44cf387000)
        libglib-2.0.so.0 => /usr/lib64/libglib-2.0.so.0 (0x00007f44cf251000)
        libQt6DBus.so.6 => /usr/lib64/libQt6DBus.so.6 (0x00007f44cf17a000)
        libpng16.so.16 => /usr/lib64/libpng16.so.16 (0x00007f44cf144000)
        libharfbuzz.so.0 => /usr/lib64/libharfbuzz.so.0 (0x00007f44cf052000)
        libfreetype.so.6 => /usr/lib64/libfreetype.so.6 (0x00007f44cef8d000)
        libz.so.1 => /lib64/libz.so.1 (0x00007f44cef73000)
        libgthread-2.0.so.0 => /usr/lib64/libgthread-2.0.so.0 (0x00007f44cef6c000)
        libicui18n.so.76 => not found
        libicuuc.so.76 => not found
        libicudata.so.76 => not found
        libpcre2-16.so.0 => /usr/lib64/libpcre2-16.so.0 (0x00007f44ceee0000)
        libzstd.so.1 => /lib64/libzstd.so.1 (0x00007f44cee1c000)
        libXext.so.6 => /usr/lib64/libXext.so.6 (0x00007f44cee08000)
        libGLdispatch.so.0 => /usr/lib64/libGLdispatch.so.0 (0x00007f44ced4f000)
        libdl.so.2 => /lib64/libdl.so.2 (0x00007f44ced4a000)
        libpthread.so.0 => /lib64/libpthread.so.0 (0x00007f44ced29000)
        libgssapi_krb5.so.2 => /lib64/libgssapi_krb5.so.2 (0x00007f44cecd5000)
        libbrotlidec.so.1 => /usr/lib64/libbrotlidec.so.1 (0x00007f44cecc8000)
        libresolv.so.2 => /lib64/libresolv.so.2 (0x00007f44cecae000)
        libproxy.so.1 => /usr/lib64/libproxy.so.1 (0x00007f44cec8b000)
        libssl.so.3 => not found
        libcrypto.so.3 => not found
        libxml2.so.2 => /usr/lib64/libxml2.so.2 (0x00007f44ceb1e000)
        libxcb.so.1 => /usr/lib64/libxcb.so.1 (0x00007f44ceaf5000)
        libpcre.so.1 => /lib64/libpcre.so.1 (0x00007f44cea7d000)
        libdbus-1.so.3 => /usr/lib64/libdbus-1.so.3 (0x00007f44cea29000)
        libgraphite2.so.3 => /usr/lib64/libgraphite2.so.3 (0x00007f44cea00000)
        libbz2.so.1 => /lib64/libbz2.so.1 (0x00007f44ce9eb000)
        libkrb5.so.3 => /usr/lib64/libkrb5.so.3 (0x00007f44ce914000)
        libk5crypto.so.3 => /usr/lib64/libk5crypto.so.3 (0x00007f44ce8e6000)
        libcom_err.so.2 => /lib64/libcom_err.so.2 (0x00007f44ce8e0000)
        libkrb5support.so.0 => /usr/lib64/libkrb5support.so.0 (0x00007f44ce8d1000)
        libkeyutils.so.1 => /lib64/libkeyutils.so.1 (0x00007f44ce8c8000)
        libbrotlicommon.so.1 => /usr/lib64/libbrotlicommon.so.1 (0x00007f44ce8a5000)
        liblzma.so.5 => /lib64/liblzma.so.5 (0x00007f44ce87d000)
        libXau.so.6 => /usr/lib64/libXau.so.6 (0x00007f44ce878000)
        libXdmcp.so.6 => /usr/lib64/libXdmcp.so.6 (0x00007f44ce870000)
        libelogind.so.0 => /lib64/libelogind.so.0 (0x00007f44ce7c4000)
        libcap.so.2 => /lib64/libcap.so.2 (0x00007f44ce7b7000)

Okay there are some more missing dependencies not just glibc.

Does the software exist as a flatpak? Nope.

This is might turn into a dependency hell situation, so I'm going to upgrade to slackware current first before doing anything else.