Happy death metal?

I don't think "happy death metal" even exist. there's, of course, christian death metal, so I think basically it's the same thing. Or not.
Hell no. The only melodic power metal band I know that uses something that resembles death metal is "Burn in Hell". Maybe the song "When Demons Awake" by Rhapsody (now Rhapsody of Fire) can have an approach towards black/death metal, or some Dragonforce songs have growls as well.

Anyways, comparing power metal to death metal is like wanting water to mix with oil, IMO.

Now, Gwar is a bit far, IMO as well. Their songs can have something of death metal, but are far from "happy" songs as well.