Does anyone know how to explain gravity powers as they're normally depicted in fiction (levitating people, creating anti-gravity areas)?
In all of these works of fiction, does anyone ever bother to distinguish between "gravity" and "telekinesis", beyond "well, its gravity innit" and "its mind powers, innit"? I cannot think of any that do. Unless you can think of a way that makes gravity powers somehow clearly distinct from telekinesis, there's no reason to call it gravity or to invoke gravitation at all other than the fact that "gravity" just sounds cooler.
Anyway, you seem to have dug yourself into that hole already, so I'll suggest one possibility for you.
gravity is the result of immense mass bending space-time
is only kinda correct. Gravity, of the sort that's strong enough to be interesting is the result of lots of mass in a small space bending space-time. A cloud of gas the mass of the sun worth of matter spread out over a lightyear isn't really very interesting. Compress it down into a space 3km across and suddently it becomes very interesting indeed.
The force of gravity is defined as $$F = {GM_1M_2 \over r^2}$$
Lets imagine a chunk of rock with the same density of Phobos, that weighs a mighty 300 million tonnes. Its surface gravity will be a miniscule fraction of Earth's... about 1mm/s2. Compact all that down inside a handy warp field such that you can fit all the mass within a 1m radius area and suddenly its surface gravity is twice that of earth. Inside the warpfield the rock still has its original density and mass, but from the outside it appears to be exceedingly dense.
By conjuring and controlling mere mountain-masses of material stored in miniscule warp fields, very strong gravitational fields can be developed, but unlike planet-sized gravitational fields their strength will drop off very rapidly so you won't end up wrecking the entire planet you're standing on. The number of these warps that you can conjure, the strength of the gravitational fields that they generate and the finesse with which a cloud of warps may be manipulated would of course depend on the skill of the magician, but all of their powers would derive from this one building block. Levitation, for example, involves forming and positioning enough warps above a target to provide an upward force that exceeds the pull of regular gravity downwards.
The Orion's Arm universe has Halo Drives, a reactionless motor inside a warpfield that tows around a ship by gravitational and gravitomagnetic effects, because the mass of the drive in such a tiny volume generates a very strong gravitational gradient. They talk more about the nature of the space-time metrics (the geometry of the warp fields) involved here, with some academic references at the bottom, if you were interested in a non-magical explanation. In that case you wouldn't be able to conjure up a warp at will, but assemble it with great difficulty ahead of time, and then when you weren't using it you'd have to park it up in orbit so it didn't tear up everything around you. Sometimes there's something to be said for magical handwavium for keeping things neat...
You may also be able to do the same trick by handwaving the warps as wormholes instead, but really it just boils down to the technobabble you prefer. And maybe wormhole metrics can't survive close proximity to intense gravitational fields. You decide.
