Is 'going to' a noun

'Gonna' is a way to spell the spoken and shortened form of 'going to'

It is used in reported speech or written dialogue: "I'm gonna (going to) watch tv when I get home."

'Going to' is part of a future verb form. It is not a noun. A noun is a name of something. Gonna is not a name of anything, unless we mean a 'goner', a colloquial slang expression in some dialects for one who has died or is beyond help.

Apart from when writing dialogue or reported speech, where 'gonna' may be an accurate way if portraying how a particular person talks, the form 'going to' should be used in written work.