Gravity varies, planet to planet; sometimes it’s so low that a world’s inhabitants are barely held to its surface, bounding along in great leaps, almost flying. On other worlds, the strength of gravity is so great that the natives are only a few centimetres tall, creeping slowly over the ground.
The Doctor avoids high gravity planets. Although the TARDIS creates its own gravity field, to step outside her door on such a world would mean certain death, crushed flat in an instant. That’s something even a Time Lord couldn’t survive.
But zero gravity, well, that’s the best fun of all.