I apologize in advance if this is a stupid or off-topic question. Since Energy can be converted into matter according to $E=mc^2$, how fast does it take for the gravity of that new matter to extend from it and start mutual attraction to other pieces of matter? Is instantaneously? or at the speed of light? or is it that the energy converted in this process already has gravity and that that gravity is only rearranged in this new matter?
The source of gravity is not mass, but stress-energy-momentum, so you are correct that
the energy converted in this process already has gravity and that that gravity is only rearranged
The change in the gravitational field needs time to propagate, though, and this does indeed happen at the speed of light.