GvG indeed was an early EQ feature on EQ PvE servers.
It was so seldom used I can vaguely recall maybe one time hearing guilds using it.
But its availability was cool. Heck, I don't even know if it still works on EQ today!
I dig Aggelos' idea with a "Going to War" voting process, it could be done in the UI under a Guild Management tool.
Realistically it would account for probably either 0% or 0.001% of your total playtime.
I am no PvP fan, FYI, but a guild going after one another, especially with today's widespread use of Ventrillo & Teamspeak might be exceptionally fun for very brief periods of time. I can just hear the chatter now. Not everybody wants to play on a PvP server full time.