I've never heard this stereotype before, but I have tried to imagine what gaming music sounds like, and it's very hard to nail that down.
On one hand, you have 8-bit artists like Prototype Raptor, and also nerdcore, like Dragonforce.
I'm not a fan of metal, personally, so if you associate the obnoxious shriekers online with hardcore and metal, I may see where you're coming from. However, I listen to an incredibly large array of music, and I play it while I game sometimes.
One experience that comes to mind is setting up playlists for different areas in WoW. I remember having The Chieftains playing as the sun goes down in Westfall.