Double Subverted: Gustav is set up as a Cold Sniper, but upon meeting him, we learn that he's a Friendly Sniper... in social situations. When he's later seen in combat, he's utterly cold and remorseless (which is possibly why he's relaxed and friendly outside of combat; it just doesn't get to him at all).
Gustav is normal. He's just from a particularly coldplace.
When heroes meet Gustav, he is very cold... but that's because his girlfriend ditched him. His mood swings rather unpredictably from that point on.
Gustav is cold towards non-snipers because he feels unable to relate to them and their experiences as opposed to his own, and vice versa. As such, he comes off as cold and distant to them. With fellow snipers, because he is able to relate to them better, he is a lot more convivial. His mood varies depending on who he's with.
Gustav is outwardly cold as ice. Secretly, he is a Warrior Poet. If he has a Sniper Duel with a Worthy Opponent he respects, he will write an elgy for him, burn it in a private ceremony and bury the ashes when no one is looking.
Averted: Snipers are just like every other soldier.
Enforced: "I think we need The Stoic for our True Companions, and it also seems to be missing characters effective at long range. Hmm".
Lampshaded: "Oh, that's Gustav. Don't try to make him change his expression - it never works".
Invoked: Gustav convinces some villagers to spread the rumor that he's a heartless bastard to scare the enemy soldiers.
"I'm not the Terminator with a sniper rifle. C'mon, I'll introduce you to the rest of the squad".
Gustav doesn't like sniper rifles, and prefers to get up close and personal.
Discussed: "Are all snipers that emotionless?"
Conversed: "Five bucks says the guy with the scoped rifle is an ice-cold hardass".
Gustav is a war criminal and an unquestioning lapdog for his side, which regularly commits genocide.