Here just jumping to the result, but I've succeeded to make the frequency counter, driven by GPS trained clock (8MHz). The original 8MHz Arduino (Arduino Pro/mini) utilizes internal RC 8MHz oscillator and I re-write fuze bit for crystal oscillator. The reason why not "external clock source" but "crystal oscillator" is for switch-ability bbetween internal crystal clock operation and GPS clock. Just disconnect X1 terminal of crystal and injecting GPS single instead, AVR works under GPS clock.
For frequency counter implementation, I utilized a library at
Just we need to change pre-scaler of register for 8MHz clock operation, as
In addition, I added rotary encoder for gate timing switch and OLED display as shown in the above image. Indeed the image shows the measurement of 1MHz generated by another GPS module. Independent two GPS module has encounter the same result ! (I am not sure if GPS is not accurate, but I guess we can believe.)