You better hope those G&P Gears will last you, one of my old G&P drop-in gearbox gear failed on me the other day (sector axle, exactly like yours). I replaced it and my MRP gears with JG gears.
Anyways, practice makes perfect when reassembling, I must have dissassembled and reassembled my MRP gearbox about 15-20 times trying to changing springs, installing new parts, install mosfet, and just to check how all the new parts are holding up inside the gearbox (looking for wear and tear on new piston specifically).
I found the hardest part in installing the gearbox is actually the darn