There are many possibilities, of course, but if you're looking for simple builds:
- Affliction - This is just a matter of deciding what conditions you want. For easy reference, I built a list of Affliction Conditions on the Ronin Army site. If you want to focus on the "keeping people from moving" side of things, then Hindered / Immobile / Paralyzed is probably the trio of conditions you want. Alternately, for modeling difficulty in fine movements, you could do Impaired / Disabled / Incapacitated. Prone makes a pretty fine 2nd tier condition to add.
- Movement - Flight. That's pretty much the easiest thing to do, describing the flight as adjusting gravity to move you in a particular direction, or to stay in place.
That said, the Batman Beyond episode you're working form was largely about intangibility with the gravity being a nasty side effect of him being unable to avoid sinking into the ground (and not having an ability to not be affected by gravity.
If I've misunderstood your question, please correct me.
Ah, and if you're looking to not have to put as much work into rolling your own, Green Ronin published a Power Profiles: Gravity Powers booklet, the contents thereof also available in the Power Profiles collection, or in the Power Profiles add-on for Hero Lab. Fair warning, Power Profiles are kind of second-degree canon. They're an official release, but they are not "Core Rules" and they were written up by different authors with some contradiction occurring as a result.