One of theoretical physics' top questions is indeed, What causes gravity? Ask
Rovelli:
According to Einstein, gravity is not a force – it is a property of space-time itself. Loop quantum gravity is an attempt to develop a quantum theory of gravity based directly on Einstein's geometrical formulation.
One way to put it is that gravity does not need energy because it
is energy, but the only thing it can do is move things from high altitudes to low altitudes. In these terms, it is also known as potential energy. But I do recommend studying loop quantum gravity, because it rules pretty hard, although it is predicated on general relativity and quantum physics with which flat earth does not play well.
Speaking of which - finite acceleration within one reference frame can never reach the speed of light, something that is brought up early and often by universal accelerator proponents when people raise that objection. However, this limit is famously part of special relativity which, being based on Newton's theory of gravity, obviates the universal accelerator idea completely. Relativity replaces Newton's theory by, in brief, combining certain variables, but it doesn't eschew gravity at all.