Last time I was in one ...
GvG Battle can happen pretty much anywhere (we did ours in the middle of Gliter)
It's anything goes PvP between the guilds so it's probably a good idea to make sure that those members of each guild that don't want to participate leave the area, as once the battle commences they can be targeted/killed by the "enemy" guild whether they wanted to be or not.
UMs need runes (if memory serves)
Guild Points are supposed to be granted, but I honestly can't remember if any were or not.
First side eliminated loses.