Physics barely has any connection with Spelljammer. Any object over 1 ton in mass creates a 1g gravity plane along its longest axis.
Not even that much. Anything longer than 25 feet generates 1g. You can walk on a tall giant like it was a planet. If you broke off a ship's mast and set it adrift, you could walk along it like it was lying on the ground. Everything, regardless of size, has an air envelope three times its size. You don't need a spacesuit because air follows you around.