If you are switching to AGameModeBase or AGameStateBase for your C++ to inherit from do both. I could not get it to work with just one. I then had to go into my BP’s that referenced any object that had these as it parent, delete and re-add them. I also had to re-specify the game state value in my game mode bp as it had been set to “none” by the editor.
I have deep scars from the editor and C++ not being in sync already. Rebuilding visual studio files, rebooting the Editor, never believing the editor reflects any changes in by C++ saves long term headaches.