it is that simple. but the workload on doing that is a bit of a pita.
In my game, i only use altered gravity in a very limited fashion and i just went around it by modifying the player physics instead but it's not really a particularly efficient way to deal with things.
another issue is that if you wish to have a real "floaty" physics, you will also need to gain access to altering the acceleration of the objects, and the acceleration is neither a variable, nor a constant, it is stored per-object in the OAM table.
there is a thread somewhere around the forum where this was discussed, and it is possible to tap into the part of the code that calculates the acceleration of objects, and change it there, but again, that's just another workaround.