As the title suggests, this is another gravity game. Here though there is a more complicated task at hand. You have to navigate your craft through 49 caves, picking up cargoes and returning them to your base. A variety of obstacles are out to stop you doing just that, from fixed gun emplacements to huge ships that roam freely around the levels. Thankfully your ship is armed too (though some enemies are invulnerable to your fire).
The weight of the cargo affects the handling of your craft, and later levels become very tricky indeed - planning out your routes based on your fuel reserves and whether to go for multiple trips or try to take as much as possible at once. One feature that makes the game easier is that whatever you achieve during a life stays completed when you respawn after a crash or unlucky encounter. Also, every five levels you get a password to start fresh from there when you next play. It's also a little more forgiving than most with regard to landing your craft (you don't have to be precisely level or moving at a snail's pace to land safely).
The one-player game is challenging and fun enough, but there are also a couple of two-player modes: Race and Battle. Both are fairly self-explanatory and immense fun. For Battle mode, think Combat but with that added element of gravity. This two-player aspect was taken a notch higher in the sequels Gravity-Force 2 and Gravity Power, but the lure of the one-player mode makes this a favourite with me.