Here is one schematic:
one 12AX7, a 6V6 and a 5Y3 rectifier.
It is pretty close to the Fender 5F1 Champ with some minor parts value changes.
This page says one of the early versions of the GA-5 was similar to Fender's 5C1 Tweed Champ:
The links don't work though
Which one of these was closer to the Gibson GA-5 you played?
Either one should be fairly simple build for a beginner.