I’m creating a setting and potentially campaign which has multiple planets and potentially space travel/battles. Not as in spaceship battles (though that will undoubtedly be present too), I am trying to design rules for low-to-no-gravity open space combat. I’m not really sure how it should work though. Do y’all have any ideas?

I recently had to come up with a low gravity effect, and in an effort to keep it simple and "feel" like low gravity what I opted for was basically double projectile ranges, double jump height and distance, and halve fall damage. Note that the doubling on jumps means that you can get twice the height/distance for the movement you spend, encouraging players to low-G jump everywhere. 😉

This gives what I felt is a reasonable sense of things being lightweight, though it's probably super unrealistic. If this is for Spelljammer then that "feels like" should be fine, though I do wonder if Wizards of the Coast might include rules of their own for this.

For zero-G you could maybe just swap the double jump height/distance and instead give creatures a flying speed equal to their normal speed, but they can only use it to travel in a straight line from a solid surface (that they can push off from)?

That's about as simple as I can think to make it; you'll want it as simple as possible if it's going to be an infrequent occurrence, if it's likely to happen all the time though then you may need a bit more detail, but you can probably just wing it and see what issues arise from gameplay?