Tricky! I think I'd just replace all the axle gears to be sure they're OK. I've seen them slide sideways after the gear cracked. You may be able to confirm this with the NMRA gauge, gauge too narrow is probably the one causing the problem.
These GP30s disassemble pretty nicely, so if I didn't see anything else, I'd take it apart and clean and resolder everything.
Are you seeing a stall or a short?
Puckdropper
