Oh, it works. I've played around with it on one of my maps. You can actually have multiple trigger_gravity's, with different settings so you can walk from an area with high gravity to an area of low gravity. You can also make the gravity negative so when you jump you get pushed up (but then get stuck on the ceiling and can't move).
The problem with all this is, you can only affect the gravity up and down, and you can't change the orientation of the player, so, sorry, no walking on walls. Though you could play around with low gravity and trigger_push's (maybe func_push, can't remember) to try to get the effect you want.