American English

Gonna
Going to
Is going to

Be going to is commonly used in informal styles.

We use be going to to talk about future plans and intentions. Usually the decision about the future plans has already been made.

We use be going to to predict something that we think is certain to happen or which we have evidence for now.

We use be going to when we give commands or state that something is obligatory (強制性的). [for example: parent to a child]

We use gonna instead of going to in informal contexts, especially in speaking and in song lyrics.

Will is often used in a similar way to be going to. Will is used when we are talking about something with absolute certainty. Be going to is used when we want to emphasize our decision or the evidence in the present.

dictionary.cambridge.org