What was on the site before the current garage and home were built? How is it causing you traffic problems if it is a single-family home? How is flooding a concern? Are you in a flood zone? If not, may be hard to argue it will cause flooding.
Good luck with the attorney. It may be tough to do much if it has been in place for 10 or more years and no complaints, especially if they are just putting their gym equipment in the garage. If they start teaching classes (even to a small group) that might be a commercial use in a residential zone which may not be allowed.