meaning - Is it common to describe "Be going to" form as Simple Future?

"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?

Answer

This one is tricky because you need to remember to conjugate the verb. Technically, it is "to be going to". But yes, they are mostly interchangeable. For example, the following two sentences have the same meaning:

I am going to have my car fixed tomorrow.

I will have my car fixed tomorrow.

But it is incorrect to say:

I be going to have my car fixed tomorrow.

In this example, the verb to be has not been conjugated.