What to do to set gravity in unity? I'm working on a game called "OXYGEN", and for the space scene. I want to set the gravity of the Controller to 0.38, or Mar's gravity.
You can set the global default Gravity vector in the Physics Manager. You can find it under Edit -> Project Settings -> Physics (or Edit -> Project Settings -> Physics2d for a 2d game).
If you have a more complicated gravity setup with zones with different gravity or point gravity sources, then you should set the global gravity to 0:0:0 and handle gravity acceleration on your own. I posted a simple script for a point gravity source here.