You can take a look at it in the source. However, it is hard-coded as “GetGravityZ()” so, as you can imagine, gravity can only be up or down.
Someone on this forum was already trying to convert this to a vector so you could set the global direction to whatever you wanted. With a quick search you can likely find the progress he’s made on it. (Edit: link)
Another solution would be to simply disable global gravity (or ignore it for your player), and apply a custom force to simulate gravity on the player. You will also need to rotate the player to align with your custom gravity, but that should be fairly trivial. Your gravity won’t be global (i.e. it won’t affect particles or other objects), but that may be closer to what you want in your case anyway.