Download Pokecenter with tutorial:
I build the gym on the server I play on as all the gym leaders always complain about other people being were they shouldn't or not being respectful.
Well, the two trainer sides close up behind the person as it opens the way to the gym. So only the people you take in with you can get in. If it's in an ocean you can watch the wild pokemon swimming while battling.
There's also two waiting areas with healers (so that people wont just stay battling making them take turns.) and a little surprise to help keep the crowd occupied. No iron, diamond, gold or redstone blocks.
Download Underwater Gym: