No rules, the softball coaches need to get some nads.
If coaches are not involved (non HS teams), then it is the somebody knows somebody schedule, then you need to find that somebody.
If it is an open gym, hey the girls can put on the leather and work out with the boys, somewhere a stand will have to happen but you are not defining on your situation clearly.
The only other way that I am aware of is, if a softball program becomes dominate every year and puts banners on the walls, league, district, sectional, region wins, they will get the gyms.