Sadly no, you can't change where the gravity is centered at. One of the few engines that could do this was the Serious Sam engine.
Just rotate the whole room...
yeah, just rotate the entire room, oh and actually the best game to be able to alter gravity is Prey. You can shoot certain sensors and flip the gravity in the room, its pretty slick.
You sure you can't flip it? I know that I've played at least one map where the gravity was at the very least altered at a certain point, if not reversed on the ceiling of one room.
You can make it so that everything moves up towards the ceiling, but you won't be able to walk around on it. Remember the part in HL1 where you turn on that giant fan and you get blown to the top of the room? Yeah, that was upside-down gravity in work, and it does the exact same thing in Source.
No, it was a trigger_gravity. Either way though, you can't walk on the ceiling using any entity.
how about a trigger_gravity or func_push or whatever to levetate the player, while simultaneously rotating the room, it would look like gravity is moving from the local floor to the ceiling. You could try to do this really fast, so that it seems like you fall to the ceiling, when in fact you were pushed up and the room rotated really fast. you don't have to push the character up much, only a few units, depending on how fast the room rotates.
how would you rotate the player, they would see the room rotate around them and the thing about moving something that large with this engine is that in custom maps at least it tends to shake and is rather noticeable
That's a good idea, artesia. As long as there isn't any physics objects in the room (since their movement would give away the illusion), it would work out great too, I think.