Gunsmithing is nothing more than a culmination of various skills.
Machining, welding, wood working, metal coating, metal fab, heat treating, tool making, engineering... Most of these skills are taught at vocational schools.
There are thousands of books and videos that teach repair and modification on almost every firearm made.
There are as many books with exploded views of firearms. A good set of skills some good tools and a good library will get you about as far as a correspondence course. You probably know lots of folks who well let you take their guns apart if you will clean it for them. Believe it or not cleaning is what most gunsmiths do because most malfunctions are from guns being dirty. But this gives you lots of hands on into workings of lots of firearms. You don't need a certificate to be a gunsmith. My old man only completed the 7th grade and after serving in WW II he was a armourer till the mid 70s and a most of the time gunsmith firearm guru most of his life. He worked on everything from M2s to Daisy BB guns and made tons of parts with saws, files and a hammer and forge. There's no magic or dark arts in gunsmithing just skills you can learn and the application of them and that will change with experience. And you don't need expensive machinery or a fancy shop my humble shop has a bench grinder, small drill press, hand drill and belt disc sander. Every thing else is hand tools either built , bought or inherited . All tucked into a 6x8 room in the corner of the garage.
Pat