Well this of course depends on what kind of gaming site you have, but I am mainly targeting PC games. Say for instance you run a World of Warcraft site or another MMORPG fan site. Just having a forum won't work because there are thousands of forums out there and whats so special about yours. To retain gamers you need to have interactive features such as tools for the game, downloads, chat, etc. These normally will build you a tight community and give you returning members. Another tip I have is if it is possible to create a server for your game, this will increase traffic alot. Many people will use your forums more often, come to your site more often, and best of all, play your server. Even if they only use forums for the server, it is still great.
Next is toplists. You can use a toplist to increase traffic. How they work is like this, you place their link or image somewhere on your page and people from your site must click on it to increase your rank. The more votes you have, the higher your rank is on the toplist, thus increasing your traffic.
Lastly is to play the game yourself. Not only play it by yourself, but jump in with your members, play as if you were one of them, not the administrator. Find out what time they play or where and join in with them. This will build a bond between you are your members and keep them coming back!
I hope this helps people

Enjoy!