As per the title: is I'm gonna get it an idiomatic, fixed expression? I cannot find any reference online where they provide a definition/explanation of such an expression.
I'm almost certain that in South Park they use it often (but locating a particular scene where they use it feels like a needle in a haystack). I think they also often use "they're gonna let me have it" (for example, "my parents" are gonna let me have it, which one of the children would say after they got caught doing something mischievous/naughty/etc. — but then, in that same situation, would "I'm gonna get it" fit?)
If I'm correct in what I think is the meaning of this expression, could you point me to some online definition?