I have tested GAP in KSP 1.12.5 and so far all the contracts work as expected. No modifications to the code necessary as far as I can tell. Only slight incompatibility is with Luna Multiplayer Server in that if you are not the oldest session on the server, you may have to ask all players to disconnect including yourself and then you rejoin alone to generate the next contract. I'll update this thread if I run into further issues as so far I've only tested the flight science contracts and the passenger plane contracts (not yet the stunts or Coast Guard).
edit: I am using Contract Configurator 2.11 maintained by KSP-RO: https://github.com/KSP-RO/ContractConfigurator/releases/tag/v2.11.0.0
edit: and Module Manger 4.2.3: https://ksp.sarbian.com/jenkins/job/ModuleManager/