Not sure which one, but either Man of Faith or War Screamer dispel allies' weapon enchantments. I was on the Jergalite Mortuary quest earlier and a level 7 cleric cast GMW on me, partway through the quest it was gone, though, and so was his, I was barely dealing any damage to some of the undead, even though his other buffs were still active. I suspect it's War Screamer, just a hunch, though.
You are aware that GMW is turn/level ?
And that there is a bug with GMW where it sometimes gets stripped randomly. Yet the fact that both of you lost it... leads me to think that you did not know it was turn/level.
Man of Faith
The divine damage that is added as a result of the Bless spell completely overrides any bless damage that you get from Divine Might.
I actually had GMW and he had MW. Though no, I didn't know it was turn/level.
I think War Screamer takes into account multi-classing or something and does it's damage/temp HP DoUbLe AmOuNt.