> "Nah man I aren't gonna" <-- this is wrong
Well, yes, because the first person singular of to be is am, not are. It works fine with I am or you are.
> I'm not gonna do that <--- looks good to me
> You aren't gonna need that <--- also looks good to me
'You aren't gonna need it' looks weird because it deviates from the stock phrase YAGNI, but it's not wrong.