Yeah, you can do this. You can trivially sell anything less that the purchase price of a settlement, which has a typical minimum of 500 gp (exactly enough to buy your powder). If the thorp (any settlement with less than 20 people is a thorp, for example your camp site. The GM can change this number trivially, though) is Impoverished, you will not be able to sell your powder without having to "settle for a lower price, travel to a larger city, or (with the GM's permission) search for a specific buyer in the city with deeper pockets."
In terms of economic power, it's not bad, but it's not really good either. Usually, we have a rogue or equivalent with Black Market Connections load up on the economic subsystem stuff and just share the money, items, and info with the rest of the group the same way we share the magic spells, info, and crafting feats with them. Since economic payout scales by orders of magnitude as you gain new abilities, rather than increasing linearly, a small investment in an economic ability is unlikely to help out your party's overall economic plan.
That's not to say you shouldn't do this! Financial independence can be helpful and your Gunsmithing feat costs you nothing. You're playing a gunslinger anyways, you can grab your 400 or whatever it is after modifiers gp/day, and at early levels that may well be a big deal! In fact, at level 1, it is the biggest deal (in terms of time-based wealth. Technically 'rich parents' and such allow for a bigger one-time money boost, but those are one time and even so will be made up for within 3 days). It's just that later on that income sort of peters out as compared to later economic sources.
As for the comparison to skills: Profession and Craft skills are very different from each other, but both are usually abysmal sources of wealth (though craft can be pretty great with careful optimization).
Profession skills don't make anyone any money. No one should ever use these for money. These are a complete waste of time, unless you have some exploit for infinite skill mods which, as far as I'm aware, doesn't exist. Even then, Craft is better. +10 to a Profession skill is doable at level 1, and nets you (check total)/2 gp per week, for about 1.5 gp/day.
Craft skills let you make stuff. Then you can sell the stuff. At base level, you make stuff for 1/3 it's price and sell for 1/2 so you get a 1/6 price profit margin. The better you are, the faster you can do this. You can make at most the better of (check total)^2/10 gp and (check total)/2 gp per week. At level one you aren't going to be doing much better than +10 to craft, and you'd be making about 6 gp/day. This is obviously not ideal as the primary source of income for your party.
As regards the prohibition of economic engagement by GM fiat, that's a playstyle thing. One way to play involves sticking to the WBL, in which case abilities like these don't actually do anything, or just slightly modify wealth by level, or something (you can see my question here about that. Unfortunately, there're no real answers posted at present :( ).
Typically, my group plays games where these abilities function as-written, which requires that the GM design the campaign with that in mind, but doesn't render play impossible or incoherent or anything. That said, using these abilities will make stock campaigns like Curse of the Crimson Throne very different.