Both your endings are natural if you want grammatically incorrect slang or vernacular. "Gonna" is not a correct English word; it is slang for "going to." You can also say "will" as in "will finish."
The really unnatural part of your message is using the word "being" in "I'm being in a class." You are using the word "am" (I'm=I am) so you don't need "being." Just say: I'm in class.
If something unusual was happening so that you were in class longer than expected, you could say, "I'm being held up in class to see my professor about the assignment. There are two people ahead of me. This is the only time he is available and the deadline is tomorrow so I can't leave."
Another way to use "being" with "I am" is like this: I'm being docked grades for handing my assignment in late.
In both cases, "being" is coupled with an action:
- being held up
- being docked grades
Something is happening. But "I'm being in class" is just being in a place. You can say:
- I've been there.
- I'll be there.
- Being in this place reminds me of...
But for some reason, we don't say "I'm being in a place." I guess the language just didn't evolve that way.