Relative sizes would affect it.
If the mare is smaller, the gelding could put more of his weight on her.
More damage tends to be done to the mares backs by the stallions teeth, if he is trying to hold on, and she is reluctant, or by his flailing front hooves, if she doesn't stand still, and he is enthusiastic but lacking in experience or skill.
A bruise from his front hooves is probably more likely than weight, unless the mare is smaller.
I would definitely get my mare out of there, and if she kicked the gelding before then, I would say he deserved it!
Is the gelding a known rig?