OK, from the sound of it you have regular gaming visitors but no hard content that an ad network can use to identify what ads to serve.
If you are seeking to make a living from the site then your best bet may be to build up the population of gamers until you can charge.
If you just want to cover costs then try Amazon combined with a bit of puff telling people that buying through your link keeps their free game on line. (You can do that with affiliate links but NEVER with ads). The punters trust Amazon and if they think the game is worth supporting then they will oblige. I do this with a site that I run for a small local charity and it covers the cost of hosting and domain name.