Both are used to describe events that are expected to take place in the future. I don’t think there are any hard & fast rules for when to use which, but I’d say that the “to be going to” fom is more often used if you think the event is going to happen shortly in the future.
This is more or less the case in languages such as French and Spanish, in which the “to be going to” construction is also found. It’s probably the same in English.
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