One of the lines in the Bane description in the SRD reads:
“Against its designated foe, its effective enhancement bonus is +2 better than its normal enhancement bonus.”
The question I bring up is where it states “normal enhancement bonus”. To me, this would imply the bonus granted by the weapon itself, not that of the GMW cast on it. If the we’re talking about a +1 Bane (Giant) Longsword who someone has cast GMW on and the wielder faces off with a Giant, I would assume it would attack at only a +3 bonus (+1 from sword, +2 from bane). In other words, the wielder either has a +3 Longsword that deals 2d6 extra damage, or they use the +4 enhancement bonus instead. Now, I realize this sounds odd, but that seems to be how the rules state, at least in my opinion. Again, this is how I interpret the rules, not saying I would rule it that way myself.
In regards to the question on whether it would bypass epic DR, I would definitely say no to that (based off opinion, not by the rules). I don’t like the idea of players using a 3rd level spell to be able to bypass epic DR. That’s just wrong, IMO.