The first "gravity" games that come to mind are Thexter, Lunar Lander, Gravitar, Thrust, Oids, and Solar Jetman.
There was an NES game that hybridized the "gravity" concept with the 2D action platformer that was so popular at the time. It was called "Low G Man". It wasn't a great game exactly, but it was interesting if you like gravity games.
Pulsar_t wrote:Thanks for the suggestions. Giantbomb lists Pixeljunk Shooter as a similar game
Subterrania is the first game I thought of when I played Pixeljunk Shooter, so I agree. I actually own a complete copy of Subterrania and used to play the hell out of it.