Get some time on modern pulsed arc machines.
In TIG welding schools, you might see a bunch of various power sources that aren't well suited for gun work - lift start, scratch-start, etc. Some of these power sources will be preferred for pipe or fab welding, but that's not what you'll be doing on a bench.
You want to seek out a HF start, pulsed machine and then practice - a lot. Practice especially on stainless steel, because if you overheat stainless, you will ruin the metal and there is no way back from the point where stainless starts to "sugar." The chromium comes out of solution and that's it, you're done, the piece is ruined. Practicing on thin stainless (300-series stainless is fine) sheets or small stainless pieces will teach you to control your arc and heat like almost nothing else will. Welding on aluminum is a useful skill in it's own right, but not one you'll use much in gunsmithing. If you can weld stainless well, then you'll find welding carbon or alloy steel pretty straightforward.
As others indicated, learn to solder well. There's low temp solders and high temp silver solders used in gunsmithing. The high temp silver soldering is more akin to brazing than soldering.