That post by Gunnar was one of the best I've seen on CGN. If you are young and you can find someone to take you on, it could be the start of a life long career. You will probably get mostly grunt work for the first year, cleaning guns, maybe installing scopes, etc. but you will become familiar with the various parts and functions of the firearm. After all, that is the life of a first year apprentice. That first year is never wasted though. You learn a great deal simply by taking a lot of different guns apart and actually seeing the various methods of operation they have. You also get a pretty good idea of what "acceptable workmanship" really is. If you open your own shop, you will probably be doing those same jobs yourself even though you call yourself "Boss".
The only hands on training I have ever come across in Canada is the school in Quebec. There are correspondence courses but after looking at them, I don't think they will give you more than a starting point in the trade or maybe get you a job in counter sales at Wholesale sports or some other chain store. Trying to figure things out by looking at the poorly copied pages of their "manuals" is not a good way to learn any trade! The other alternative is the video courses which give a great deal more information and you get an over the shoulder view of the whole process. The down side is that most of these courses are make/model specific and the ones that do offer the full gunsmithing video course don't like to ship outside of the U.S. Of course, there are gunsmithing schools in the U.S. but you are looking at 2 - 3 years of living in the states and the expense of post secondary education.
If you are older like me (50) then you likely have some experience, training or at least a collection of tools to help out. Those transferable skills that everyone is supposed to have can come from unlikely sources. I used to work in diesel fuel systems until injuries made me look at career choices. But looking back at my trade, I know that I'm good with small parts, I can follow the mechanical functions and I have had experience with lathes and milling machines from making parts for antique or obsolete equipment. I've also had experience with cold and caustic blueing from rebuilding injectors and have worked with most normal shop and welding equipment. BUT perhaps the most important thing I've learned is that knowing what the customer expects to get back and delivering that to them is the best way to avoid being called an a**hole.
No trade is easy to break into and the first couple of years are the hardest but if you can push through those lean times, be willing to learn, and to talk to the customer you have as good a chance of making it as anyone.