You only need one weapon part for the achievement, but each one is helpful to you throughout the campaign. There are 10 total, and if you do get them all you'll also get these achievements in the process.
A couple of weapon part are also behind locks, so you'll also make progress on RE: Master of Unlocking in the process.
Two videos below. First is all collectibles, which includes all weapon parts. The second is for all achievements in the game, which of course also includes Gunsmith. Take your pick and I hope it helps!
Lock 1, Weapon Part 1 - 3:20
Weapon Part 2, Hip Pouch 2 (Need all Jewels) - 14:51
Weapon Part 3 - 16:28
Weapon Part 4 - 16:55
Weapon Part 5 - 21:39
Weapon Part 6 - 24:22
Weapon Part 7 - 30:37
Lock 15, Weapon Part 8 - 32:03
Weapon Part 9 - 38:23
Weapon Part 10 - 39:21
Lock 1, Weapon Part 1 (Gunsmith) - 5:44
Weapon Part 2, Hip Pouch 2 (Power Stones) - 17:23
Weapon Part 3 (Nemesis Down) - 19:29
Weapon Part 4 - 19:56
Weapon Part 5 - 25:48
Weapon Part 6 - 28:34
Weapon Part 7 - 35:19
Lock 15, Weapon Part 8 - 36:45
Weapon Part 9 - 47:13
Weapon Part 10 - 48:10