I like the idea of a gavity block, but I think it should react to a redstone signal rather than need to be in a sealed room. Gravity and oxygen are independent things.
You place the block and give it a redstone signal, and it'll pull entities up to X blocks above it down towards it. I say X blocks so that players would be able to play other blocks above it to make things look pretty. Think of it as gravity plating under a floor. Having the block use a redstone signal rather than the seal mechanic would let players control gravity within rooms with or without oxygen. Turning off/on gravity while players are entering or leaving through airlocks for instance. Imagine you entering your space station after a spacewalk. The outside airlock closes and before the inside one opens, your gravity plating is turned on pulling you down to the floor. Once you touch down, the inner airlock opens and you can walk into the rest of your base.
Meanwhile you might have a machine room where you'd want to turn off the gravity to go replace some piping, but would still want to breathe.
It would be cool to also have a second tier of gravity block that would let the player actively decide the amount of gravity (or lack thereof).
A third tier could also let the player decide the amount AND direction of gravity. That could be used in neat game rooms or PVP arenas (think Battleroom from "Ender's Game").