Given that the Campaign Clarifications (and Word for word in Additional Resources) says:
Page 103—Gunsmithing does not grant the ability to craft firearms, ammunition, or black powder. Rather, it allows the purchase of bullets, pellets, black powder, and alchemical cartridges (with 1 rank in Craft [alchemy]) at the listed price, but does not grant a discount on the purchase of any firearm. Resold items gained through this feat are worth half the actual cost paid, not half the regular market value for the item. No PC can purchase a gun without this feat, even if they possess the Amateur Gunslinger or Exotic Weapon Proficiency (firearm) feat.
And the season 8 Guide in Appendix 1-Character Creation Section 8- Skills says:
Ranks assigned to Craft skills are used only to perform
Day Job checks, not to craft mundane items, with the
• Alchemists can use Craft (alchemy).
• Gunslingers can use Craft (gunsmithing).
• Investigators can use Craft (gunsmithing).