It's very hard to MIG weld bicycle frame tubing -- it's just too thin, and you don't have enough heat control. Even if you could make a successful weld, MIG is messy with lots of splatter that you'd have to tediously grind/file off. It's just the wrong process for bike frames, kind of like trying to give yourself a haircut with a lawn mower.

Since you've developed some proficiency with gas welding, why not try fillet brazing?