As far as getting access to every player in your game, if you are on the server, then you can just iterate through the PlayerControllers, as it will have one for each. Alternatively you can get the Game state and get the PlayerArray, which will give you their PlayerState.
To set the gravity you need to edit your World Settings.
Borrowing from this post :
AWorldSettings* MyWorldSetting=GetWorldSettings();
MyWorldSetting->bGlobalGravitySet=true;
MyWorldSetting->GlobalGravityZ = 900.f;