I am going to mention tough ones - turning down work, and being a businessman.
The key is to have a work rate, just like your plumber does. You probably need to charge at least $40-60 an hour (check how much your plumber charges) or you will go under fast.
There are some jobs that you will have to just turn down, even at the risk of some harsh words from a would-be customer, because they can't feasibly be done or because the cost will exceed the value of the gun.
The most common are old H&R or Iver Johnson revolvers, old .22 rifles, and old shotguns; these are almost always totally worn out, with broken springs and bent, broken or filed parts from previous attempts at repair. You can easily spend days trying to get those junkers working (and maybe fail) but you just can't afford to and make any kind of a living. So you turn them down.
You should decide early on if you want to have a shop and deal directly with customers, work for an established store, or take work from local gun dealers.
If you go with the shop, don't spend time BSing with customers. Hire a minimum wage kid with enough gun smarts to tell the guy with the Stevens Crack Shot that it can't be rechambered to 300 Win Mag. Your job is to gunsmith, not chat about who got the biggest buck last season.
If you are going to run your own business, I strongly recommend taking a small business course (often offered by community colleges). The finest master gunsmith in the world, who can file a precise 1" cube, won't stay in business long if he doesn't know how to run a business. If you approach the job as a hobbyist, doing free favors for your friends, tinkering with old worthless guns, and spending your time chatting with gun guys, you will go under. Not might, will. You can hire a bookkeeper and/or an accountant and/or a tax guy but they are like your tools - if you don't know how to use them, they are little help. (I knew of one good gunsmith who kept his "accounts" on scraps of paper he kept losing - maybe that worked in 1820, but it won't work today. He went out of business when the IRS got after him for income tax and the state wanted its sales tax.)
Some schools do cover this kind of stuff, but most tend to deal with the technical aspects of machine work, etc., and devote little time to the details of running a business.