Gentlemen clubs are where men go to see topless or nude women dance for tips. You can also purchase private lap dances for approx. 20-40 dollars. And lots of alcohol. If you suspect your husband is going to these clubs, perhaps you should go with him. This is what I do. It adds some fun to the relationship and then it doesn't feel like he's doing things behind your back.
In more direct terms, a "Gentlemen's Club" refers to a strip/exotic dance club. A Gentlemen's Club can have bikini, topless, or fully nude dancers, depending on the local and state laws.