Close its mouth at the speed of light?
Close its mouth at the speed of sound?
Close its mouth faster than the speed of light?
Also… wouldn’t it be heavier than the galaxy itself? So it would create its own gravitational pull that’s like extremely strong?
There’s basically no way to answer these questions using real physics I’m afraid.
It definitely can’t close it’s mouth faster than the speed of light. Yes it would have a strong gravitational pull, almost definitely so strong that it would just collapse into a black hole and not be able to exist. If it weren’t that dense, then it would basically just be a big diffuse gas cloud that couldn’t do much anyway, it would basically have to be a proto galaxy to not collapse into one.
If it’s using magic to exist, then anything is on the table.