I made the GM Custom from "SD Gundam G GENERATION-0" No. 18.
I made a few slight tweaks to the build (I'll write about that later), but it's not really a modification, so I'll treat it as a "simple build" piece.
Incidentally, this kit is a different color molded version of the Gundam NT-1 (Alex) from BB Senshi No. 37, with the addition of the head, arms, rifle, and shield parts for the GM Custom.
I also had the BB Senshi No. 37 Gundam NT-1, so I compared the contents. It's definitely based on Alex.
Now, about the build...

G ZERO No.18 GM Custom | クラフト's Gunpla works