I’ve found two interesting gravity-based games.
One is MindScape, which is a simple platformer, but if you walk up against a wall, or fall toward it, your gravity will orient as though that surface were the ground. It’s nifty. You can play MindScape here.
The second is the aptly-named Gravity Game. It, too, is kind of a platformer, and also uses gravity as a gameplay element, but is more of a puzzle game than MindScape, which isn’t too difficult. In the Gravity Game, you are a small circle, and your gravity, falling speed, ect change based on what color you are. You change your color by hitting color beacon-things, and your goal is to navigate these clever mazes to obtain these light-post things called Spindals. Gravity Game found here. (I’d recommend this one)