Well, I'm not an experienced enough photographer to offer you technical advice in this area, but I have worked a lot of weddings in other capacities, including more than a few in Gymnasium/Multi-Purpose rooms.
Before you go to too much trouble trying to figure out how to get flattering photos of the wedding party and guests under the institutional fluorescent lights you typically find in a Gymnasium, I would suggest trying to contact the wedding planner o find out what kind lighting is actually going to be used at the event. In most of the weddings I have worked in this kind of venue, the event planner brings in at least some of their own lighting equipment. Photographic concerns aside, institutional overhead fluorescent lighting isn't very flattering and doesn't generally provide a very festive atmosphere. So most planners will avoid using it if the budget allows. Temporary carpeting or other floor coverings, as well as wall drapes, etc. are also commonly used to dress up a space like this, and these could also change conditions -- you won't have to worry about bounce off the hardwood gym floor if it's covered.
But if it's really a budget affair, they may be using the institutional lighting. With a bit of luck, maybe you can get in touch with someone who will tell you what the plans are in this area.
