The big bang theory is the claim that our universe can be described by a solution of Einstein's equation with an initial singularity. Such a solution describes a spacetime in which every event has a time coordinate t>0. There's no t=0, and the big bang isn't an event in spacetime, it's just a name for the mathematical limit t→0.
In other words, the best theory of space and time that we have strongly suggests that it isn't possible to define the words "before the big bang" in a meaningful way. You may think that if there was a time t=0.5 seconds, there must have been a time a second before that, but that line of reasoning isn't valid. It's based on our intuition about the properties of time, but the fact that general relativity makes much better predictions about results of experiments than any theory that agrees with our intuition, proves
that our intuition about time is wrong.
The best theory of gravity is general relativity. It describes how the properties of spacetime are related to properties of the matter in spacetime. So the idea of gravity without time doesn't seem to make much sense.