Check your local community college for basic machining classes and basic welding classes. Unless you just plan on doing basic parts replacements, you'll need to learn how to set up and run a lathe, a milling machine, a O/A torch, a Mig welder, a Tig welder, and possibly a surface grinder.
I worked for a gunsmith in my college days, and he was very limited with only a lathe, a grinder, a buffer, a set of bluing tanks, a couple of benches, a bullet trap, and a bunch of specialty hand tools. When he was ready to retire, I was considering buying the business. I asked what he would add if money was no object, and he said a milling machine. He sent his milling work to a local machine shop. Even then (1983 or 84) we valued what he had at $20k on the used market.
If you just want to do gun repairs (replacing broken parts with new factory ones, changing sights, reassembling perfectly good guns the owners took apart "to clean", bedding actions, installing aftermarket stocks, etc) you can do that pretty cheaply. The local range has a "gunsmith" that rents space there. He charges around $75 to do such work, and is backed up six weeks. He only has hand tools, but did graduate from the gunsmithing school at Susanville.
Several things work against gunsmiths these days. First is the changes in technology over the last 50 years. It used to be a mystery what happened behind the gunsmith's bench, but now many common things have half a dozen YouTube videos to show you how. Guns used to be hand-fit works of art, requiring skilled craftsmen to repair them, hand-fitting parts. No wonder the gunsmith who taught me hated Glocks... Now, anyone with an interest and a set of hollow-ground screwdrivers can do many of the things it used to take a pro for. And 30 years ago, NOBODY but a gunsmith owned a sight pusher...
Still now, as then, if you give good service you can build a business. I spent the last two weeks in August sitting on a stool at the front counter, greeting the guys with a barrel in one hand and a grocery bag in the other - desperate souls, willing to pay to have someone put their shotgun back together before their wife found out. The last Saturday in August (dove season opens Sept. 1) was a madhouse - I was charging $20 a gun, doing 4-5 per hour, and had a line waiting all day. I did 52 guns that day, and split the proceeds 50/50 with the shop (I was casual labor) so I left with $520 in my pocket. Not a bad day for a college student in 1981. Too bad those days were few and far between.
The downside can be steep, though. You screw something up, and you have to make it right. You tell a customer it will cost $79+ parts, then hit a snag and spend four hours on an hour job, and it stings a bit. You slip and damage the finish on a $20K shotgun....
If having mechanical aptitude, a sharp mind, a steady hand, and $1,000 in tools were all it took to make a living as a gunsmith, there would be a lot more gunsmiths.