First off- the lower area is predominantly for campers and group campsites. Stay up on the hill if you would prefer to be away from the campers. The hill sites have basic portapotties, which was sufficient for most of our needs.
The bathroom facilities down below have running water, toilets, and showers. It was chilly while we were up there, so I can’t report back on how the showers were.
We go to GM often, but normally stay at a cabin outside of town to save money. This was a great way to be in town on a budget. Walking to and from town was a nice ritual, but you will not regret bringing a bike to speed up your commute into town.