kapusta: anyone who takes a fee, even if they say it’s for tzedaka, is not a gemach.
A gemach should be free for those who need. The people coming need the help, so why are you asking them for tzedaka? It should be called a rental, not a gemach. The only money they should be collecting is a deposit on the items, which is returned when the item is given back.
You can ask for an optional donation, but if it’s required, it’s not a gemach.
Oh, a gown gemach can also require that the gowns be dry-cleaned.