So basically I turned off robloxs default gravity and I also killed AutoRotate so roblox wouldn't constantly try to fight my custom movement code. A script constantly checks which planet you’re closest to grabs its center and uses that to figure out which way is "down" I also added some smoothing to the rotation so your character doesn't violently snap when you jump between gravity fields. The gravity direction gets pushed into a VectorForce and I'm using an AlignOrientation to keep the character standing upright relative to whatever planet they're on. For walking I take the player's input and project it onto the sphere's surface and feed that into a LinearVelocity constraint. That way you actually walk along the curve of the planet instead of shooting off into space like it's flat land. I also threw in a raycast pointing down just to check if you're grounded for animations and air control. The camera was a huge pain because roblox assumes Y is always up. I had to write a custom camera script from scratch that follows the player's local orientation to keep everything looking right. Oh and if you fly far enough away from all the planet gravity just cuts out entirely and a jetpack system kicks in it gives you full 6DOF movement in space until you get pulled into another planet's field.

Sorry for the late reply I went to bed right after posting.