Planetary system dynamics can get pretty exotic, but "falling up" is probably restricted to anti-grav technology or magic. Since - by definition - "up" is the way you don't fall.

Of course, given ultra-tech or magic, there's nothing stopping you making small zones where the gravity is inverted compared to most of the world around it. This is one of the most fun things a GM can do.

Unless you're a player, in which case it's one of the most annoying things a GM can do.

Transcript

GM: ... Okay. R2: you land on the leaf litter ejected from your wheels, absorbing 1 point of damage. And the reduced downward momentum from ejecting it somehow slowed your fall enough to reduce another point. You take 10 points of damage.
GM: Han: you flail wildly and land awkwardly, taking 17 and a lingering sprain.
[SFX]: SPLAT!!!
GM: Luke: you land uneventfully and take 12.
GM: 3PO: you land on Chewbacca, who cushions your fall. Take 6.
GM: Chewie, you take 14, minus 2 for furry padding, plus 5 for 3PO landing on you.
Han: Is all this adjusted for gravity? This is only a moon.
GM: ... Of a gas giant.
Han: Exactly! Even more gravity pulling the other way. How do you know we won't fall up?
GM: You helped me design this system.
Han: Oh, right.
Chewbacca: When you put it like that, one could say you are the architect of your own downfall.