[SPOILER ALERT!]...
I defeated him first the fair way, with Razit's Spear, then saved, loaded the file before the fight and defesated him again by provoking him into submission... Dude, that was so satisfying! :V
(There are also spoilers in this reply!)
It is worth noting... once you beat him, you have several options on how to deal with him.
He remains /quite/ pissed off at you if you fuck him, a fact he makes extremely clear... but mellows a bit if you have a joint jerk-off with him. The tone is very different and could conceivably result in him being more for realsies friendly in the future despite you beating him.
I really /like/ that interaction as it's so clearly fitting with his personality. He's a lot like Logan in that respect - he has emotional baggage he's carrying with him and doesn't really want to open up, but if you're willing to put in the mileage, then just maybe...
Definitely true - I didn't mean to imply that he and Logan were the same, haha. Nor that Gunnar is the most, um, stand up moral individual :P
But hey. Maybe Gunnar just needs someone he can actually trust in!
(Or, alternately, maybe he's just always going to be a jerk, haha)
(Also I'm curious what precisely his connection to Scar is, haha, but that's neither here nor there at the moment.)