I was being in a class. The professor was lecturing. A friend sent me a WhatsApp message
What are you doing? I need your help.
I replied
Is it Urgent? I'm being in a class which is gonna finish in 20 minutes.
Should I have said it this say?
... which is finishing in 20 minutes.
Is it more natural in this context?
I'm in a class which is gonna finish in 20 minutes.
...which is finishing in 20 minutes.
are both grammatically correct and both used. More succinct is
I'm in a class which finishes in 20 minutes.
But I might prefer
I'm in a class which is over in 20 minutes.
I'm in a class which has 20 minutes left to go.
*I'm being in
sounds terrible in standard English, outside of India maybe.
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:
Something is happening. But "I'm being in class" is just being in a place. You can say:
But for some reason, we don't say "I'm being in a place." I guess the language just didn't evolve that way.
