Last week you could hit rank 2 with NF buff. Gunsmith had new weapons on Tuesday and another new set yesterday. Plus you can pick things up randomly which give you rep with the Gunsmith. I picked up something last night called GEO-67XX (or something weird like that) which later turned into a Legendary Fusion Rifle that I had to turn in with the Gunsmith for 200 Rep. Also, as part of the Chaperone exotic quest I believe (cant confirm myself) that you have to buy a Donation for 15,000 Glimmer which also gives you rep with the Gunsmith. I think Rank 3 is very possible with a bit of luck.