It certainly could be a separate trade. Jost Amman's 1561 Book of Trades ( Ständebuch) differentiates gunsmith ( Büchsenmacher) from not only blacksmith ( Schmied) but from gun stocker ( Büchenschaffter). Obviously, though, there's some overlap in skills and tools. A smith is someone with a hammer ( a smith smites) and both a blacksmith and gunsmith use it with an anvil to work iron and steel. Both would know how to forge weld, how to harden and temper, how to file, punch holes...However, the gunsmith is specialized, and specialization in tools and skills was the way craftspeople could both increase their production and do better quality work- which meant, better pay. Blacksmithing itself had specialties. Amman's book showed a farrier, someone who would make and fit shoes to horses. But there were smiths who also specialized in making nails , or chain.

The hazard of a specialist was that there would not be enough of a market for the specialty. The maker of iron chain might be well-employed in a seaport town- but maybe not out in the country. Transportation was not always easy, and risks of damage or theft could be high. Crafts needed suppliers: the dyers of 16th c. Flanders had good access to English wool coming across the Channel, the iron workers of 18th c.Birmingham had the advantage of access to local iron and coal. And, craft knowledge was passed by apprenticeship, and apprenticeship tended to build family connections among craftspeople. Specialization tended to also attract specialized customers and so create a specialized marketplace. Because of all this, crafts often concentrated in certain places. Birmingham iron workers became the source for barrels and locks to the New Land Musket in the 18th c. If you wanted to buy scientific instruments ( like astrolabes, drafting instruments) in the 16th-17th c., you'd go to Nuremberg.

But in places with sparse population, there would be a poor market for specialties and so also not much competition from specialists. The lone gunsmith in a vast wilderness could indeed find himself blacksmithing to earn a living- or a blacksmith could find himself having to do gunsmithing. That happened in the North Atlantic colonies often enough. The Gettys in earlier 18th c. Williamsburg Virginia would turn their hands to almost anything- gunsmithing, casting candlesticks, making cutlery; even doing some silver work- which is what Colonial Williamsburg has their recreated shop doing now.