Hopefully, someone who actually lives in an English-speaking country will be able to give you a more accurate answer. However, as far as I know, 'be going to' is not generally used in the main clause of conditional sentences; 'will' or 'shall' are preferred instead. Probably that's because 'will' is the closest approximation to a 'neutral' future, while 'be going to' implies either that an event is already on its way or 'intention' in the future.
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