Gravity Force 2 was a freeware game from two bedroom coders. It was featured on an Amiga Power coverdisk and championed by the magazine. They requested new features in the game and the updated version was then called Gravity Power.
It wasn't sold in the shops, so the original being sold might be a coverdisk. There is a Gravity-Force (http://hol.abime.net/?id=2789
), but the coders of Gravity Force 2 have nothing to do with the original. The sequel is probably just a homage.
There is also a level editor.
EDIT: I think that there was a also shareware version of GF2.