I think you are mixing a few different things here. Some are pronunciation, some are slang, and some are regional variations.
For example, I SAY "gonna" in rapid speech, but I only write it in total jest (Oh yeah? Sez you! Whatcha gonna do about it?) or as dialogue. I never write "I'm gonna look into that for you" -- same with "wanna." I'm sure I say it, but I'd rather have my fingers broken then type it they way I see people use it online all the time now. That's a pronunciation issue, as is your "whatcha gonna" example.
Slang are words like "couch potato" (note it's couch, not cough) or "screw up."
Regional variation are things like soda vs. pop. (I say soda, my husband from the Midwest says pop.) Other examples include sack vs. bag, faucet vs. tap, etc. These aren't even colloquial, I think - it's simply the name things have depending on where you live.
So what is it you're trying to accomplish with your question? It will help me answer your more effectively.