Given that the holodeck can simulate different planets and is also used for training purposes my guess is that it has independent gravity plating and possibly gravity plating in all walls and ceiling as well as the floor.

There is at least one episode of Voyager where Torres goes orbital diving and stops mid dive and then stands up straight in a way that implies that the holodeck helped her reorient or at least gradually turned the normal back on. (IIRC the episode is called Extreme Risk)