Let me put it this way: is the statement "there's a phenomenon called «gravity», experienced by all massive bodies, that accelerates them in relation to other massive bodies" epistemically true?

Scientifically, maybe? Because that's what the scientific method is, best approximations given the knowledge we currently have.

But let's assume yes for the purposes of this reply.

If truth was subjective, the answer would be "true" or "false" depending on the subject.

And context.

Same subject different circumstances, different gravitational forces.

For those whom the answer is "false", this means they would not experience the phenomenon, even in situations other subjects would; e.g. near Earth.

That's a binary interpretation of a non-binary system.

But again, for the purposes of this reply, sure.

That implies they'd have at least some control over experiencing gravity, because they could simply say "it's now true for me" and fall, or "it's now false for me" and stop falling.

There's a big assumption there that this is a binary.

Gravity control, doesn't have to be binary.

It doesn't even have to be direct, they could achieve the same effect by increasing or decreasing mass.

But let's say it's magic, direct control.

In an objective system where gravity exists it would conceptually be possible to control the level of gravity acting upon yourself without turning it on or off.

In a subjective system where gravity could exist or not depending on subject and context, the same is true.

Which brings me back to:

There’s no reason gravity control requires a subjective truth.

Emphasis mine.