This isn't 'tricky' at all! In fact, it is basic standard English. If you look in any grammar book or on any decent English language site, you'll find a list of the four standard ways of referring to a future event: will leave, are going to leave, are leaving (present continuous) and leave (present simple). The difference between these forms is the level of certainty, with the present simple being the most certain and the most fixed. We sometimes refer to this as the 'timetable' future, as we often use it to refer to schedules, especially travel arrangements.

'gonna' is not real written English. Your Chinese English standards seem to be over-influenced by 30-something slang.

只想給一個溫馨的提醒，最好向給您批改的作文表示感謝。西方人很注重這個道理。 也許您想閱讀這個鏈接https://www.italki.com/discussion/201704 有些英語母語者對不懂道謝的會友表示抱怨。