Completing all the weapon bounties, as you know will get you close. You gain additional reputation with the gun smith by completing your class specific weapon quests that appear to be unlocked at rank 2 and rank 3 (for hunter Still piercer a legendary sniper rifle and The Ace of Spades at rank 3, and exotic hand cannon) each portion of the quest lines I believe grant 75 reputation points with the gun smith. Furthermore, while out slaying minions of the darkness, you may be fortunate enough to stumble upon a legendary general pick up (looks like those little pyramid shaped drops that usually are glimmer boosters or money consumables turned in at the cryptarch) that has the icon of Pocket Infinity with some odd name and numbers in it. Each of these grant exactly 200 reputation points to the gun smith when turned in. At the start of TTK, they were dropping fairly often for me, I got at least 4-5 of them during the first few days. Now though, I think bungie ninja patched them, and they have not dropped for me since. This is how I'm 295 rep points away from rank 4 with the gun smith. GL and happy hunting.