There are several methods brewing for hooking up a keyboard. The one most in line with the system is WattSekunde's four chip design for a matrix keyboard interface. But such keyboards might be hard to find, while PS/2 keyboards are still easy to get. But their protocol is convoluted and it is unclear how to handle that with a few simple logic chips. So... we cheat!
PS/2 is simply too modern, and we need a time machine to translate its fast and complex signals to the simple signals that the Gigatron will understand. The ATtiny85 will be our Babel fish.
To get some idea: the ATtiny85 must be configured to run at 8 MHz to do its magic. 1 MHz won't be good enough to catch and transfer simple key strokes. Here is the system in action: