[PrO.GameR]

As Vince said if you trust your developers just move the code to a private git repository (even tho I believe you can restrict accesses of each users git is almost useless if you want everyone to have what they created and need only)

Or you know, maybe you can teach them how to hook callbacks so they can get their systems working without touching other systems, it's always easier to work on modular gamemodes as each system is in 1 file, isolated and easier to look at