Gotta : have ( has ) to do something .
Ex : I gotta go my own way . ( Gabriella in HSM 2 )
Gonna : going to do something .
Ex : He's gonna leave his school soon .
Ain't : contraction of " am not " . Sometimes it is used as a contraction for are not, is not, has not, and have not .
Ex : It ain't fair at all .
These words are acceptable for use in speech but in writing there is no acceptable substitute for the stilted am I not?