I would say that it depends on the server, like Cressy said. Not every server is the same neither they categorize what a GM should do as all do.
It can be something basic as handle problems that players have to be something "complex" as a dev.
Some people like to adress tasks/tags to them according what they do. Some will classify GMs as just GMs and have a list of what they do in the forum or website while some will say what they do by saying that this is Event GM and do Events, the other will be Support GM and just run the Support Center, the other dood will be the I do food while you watch and slurs, etc.
If someone wants, they will even do a GM be like an admin and just pay them a monthly fee to do their work ...