Changing one's own gravity to gain an advantage over other players is easily against the rules, imo.
In the past, I have manipulated my own gravity, but not to prevail over other players whose gravity is standard. It's used, sparingly, on fun occasions...not to help me beat out the other players.
Look at the intent. Just changing gravity is and of itself not necessarily abuse (except when you do it and the whole server dies, every round, because gravity is 9999....evil...:P). There must be a context to the abuse.
If what the OP said is true, and gravity was changed so humans could be tagged by a zombie, then yes, that qualifies as abuse imo. If it was meant as fun and in a joking way, that lessens it only slightly. I'm sure there were people who'd be pissed about getting tagged because an admin changed his own gravity to do so. A demo is needed so the truth can be determined. Without it, there is no case.