Are there already rules for this somewhere? I want to look at any official rulings before I make things up. If there are no rules for this already I would like to hear how other GMs have handled this as well.

Difficult terrain.

works for me.

AoR core pg 227.

For a "standard" 0g environment I assign it an Unwieldy 3 rating, maybe 4 if there's a lot of loose stuff floating about. What I term "negative g" environments are Unwieldy 5 or 6 and treated as Difficult Terrain. High g environments are Cumbersome, usually 3, but extremes of 4, 5, and 6 are also treated as Difficult Terrain.

I alao use Unwieldy 3/4 when fighfing erupts in crawl spaces, and Cumbersome 3/4 when underwater.