"Be going to" form is called Simple Future in some English Grammar websites and books.
Simple Future has two different forms in English: "will" and "be going to" English Page
The Simple Future Tense has two different forms in English, will and be going to. Contextual English
I had discussion with a very good English user (I don't know if she is a native speaker or not) about this form. and she said she had never heard of "Be going to" described as Simple Future.
So, Is it common to describe "Be going to" form as Simple Future?