It is a feat not a skill.
Gunsmithing
You know the secrets of repairing and restoring firearms.Benefit: If you have access to a gunsmith's kit, you can create and restore firearms, craft bullets, and mix black powder for all types of firearms. You do not need to make a Craft check to create firearms and ammunition or to restore firearms.
Crafting Firearms: You can craft any early firearm for a cost in raw materials equal to half the price of the firearm. At your GM’s discretion, you can craft advanced firearms for a cost in raw materials equal to half the price of the firearm. Crafting a firearm in this way takes 1 day of work for every 1,000 gp of the firearm’s price (minimum 1 day).
Crafting Ammunition: You can craft bullets, pellets, and black powder for a cost in raw materials equal to 10% of the price. If you have at least 1 rank in Craft (alchemy), you can craft alchemical cartridges for a cost in raw materials equal to half the price of the cartridge. At your GM’s discretion, you can craft metal cartridges for a cost in raw materials equal to half the cost of the cartridge. Crafting bullets, black powder, or cartridges takes 1 day of work for every 1,000 gp of ammunition (minimum 1 day).
Restoring a Broken Firearm: Each day, with an hour’s worth of work, you can use this feat to repair a single firearm with the broken condition. You can take time during a rest period to restore a broken firearm with this feat.
Special: If you are a gunslinger, this feat grants the following additional benefit. You can use this feat to repair and restore your initial, battered weapon. It costs 300 gp and 1 day of work to upgrade it to a masterwork firearm of its type.
TL/DR: If you have the gunsmithing feat, it's automatic. No skill roll needed.
It is a feat not a skill.
Actually, it can both.
Firearms can be crafted using Craft (Firearms), since they are not "exotic melee or thrown" for Craft (Weapons). DC should be at least 18 (on par with other exotic weapons), with combination and double-barreled versions, as well as pepperboxes, culverin, and double-hackbut, with higher DCs (I'd say DC 20 for combination, plus an equal Craft (Weapons) check for the melee weapon portion, and double-barreled versions, DC 22 for culverin and double-hackbut, and DC 24 for pepperboxes). Of course, using the Craft skill is slow, only accomplishing check x DC in silver pieces per week; on the plus side, the cost of raw materials is only 1/3 of market price.
Using the Gunsmithing feat, crafting firearms does not require a check and can be accomplished much more quickly (1 day per 1,000 gp of parket price, minimum 1 day), but the cost of raw materials is 50% of the market price.
That don't seem fair or logical. I'm mean its like the chicken or the egg deal. Which came first the feat or the weapons. Someone had to make the weapon before anyone couldhave ever used feat. I mean there had to be some mad genius who kept blowing his lab up til he perfecected the gun n he didn't have nothing but skill n schooling also if u can only craft one to obtain one at half cost then why would there ever be full costs? Does that mean if I make one I can sell it for double?
I'm not sure what isn't fair or logical about it. The feat makes making a gun MUCH quicker and easier than using the craft skill. Dragonchess Player points out that the Craft skill should work, too.
To answer your questions:
"why would there ever be full costs?" Because PCs spend more to buy items than they receive when selling them.
"Does that mean if I make one I can sell it for double?" No - you'll get half the list price for it. The crafting skills and feats (both mundane and magical) are to help you get equipment and weapons you want at a discount by spending a feat instead of gold. It's not so you can set up a side business.