Kinda confused... Where do I start?

[donnydarkoIRL]

Understood, but I want to be a fighter, I just need to gain weight and mass to do it.

By gaining weight you are sacrificing a huge natural advantage in boxing competition; in terms of height and reach.

I'm sorry, but I don't know what you mean?

If you fight at 140lbs, thats light Welterweight. The average Light welter is 5' 6''

You will have a huge height advantage over your opposition.
Your arms will also be far longer allowing you to pick him off with your reach.

If you go up in weight, you'll fight taller guys, that can hit harder and like to hit.