Some GameMode functions have changed parameters, for example SpawnPlayerController has an extra NetRole parameter:
virtual APlayerController* SpawnPlayerController(ENetRole RemoteRole, FVector const& SpawnLocation, FRotator const& SpawnRotation);
With these ‘override’ errors I found it easiest to open up GameMode.h (in Visual Studio search in the solution explorer) and compare the parameters of GameMode and the subclass.
Some Receive___ functions have been made BlueprintImplementableEvents so cannot be overriden in C++ anymore, for example: ReceiveTick, ReceiveHit, ReceiveDrawHUD. They were probably intended to only be overriden by blueprints. You should use Tick, NotifyHit and DrawHUD for these.