If gravity is the only force acting on you, why
aren't you moving down right now?

Because the force of gravity depends on the size of the planet, so a smaller planet will have less gravitational force than a larger planet. So our gravitational force is not as strong as a bigger planet so we can still stand up and jump normally but on a smaller planet we could jump higher and on a bigger planet, we could not even be able to jump at all.

not too sure thinking all accelerations are pointed down