Gunmar is an NPC in Skyrim: Dawnguard
He is a Nord Hunter. He is also a Smithing Trainer.
You have to recruit him to join the Dawnguards [1] during the quest A New Order
Gunmar asks you to help him slay a Cave Bear before he agrees to hear Isran out.
Gunmar can sell you Armored Frost Trolls for 500g. These trolls become your followers and help you fight monsters.