Research gap, I understand it as a field, subfield, mechanism, that has been under-researched and there is space for improvement.
For example, object detection using telepathy might be under-research, so there is a gap in the research of that specific field.
The research question seem to refer to a more specific hypothesis, which in cases, it can coincide with the gap. For example, can telepathy work to detect an object, or can telepathy work under these or those conditions. In my understanding the first could also be a gap, the latter is more of a question.
Then the main question could be, "can telepathy work", while the subquestions could be, "can telepathy work also while you sleepwalk", "can it work if you are drunk", "can it work if you are tired".
Just to avoid misunderstanding, I'm not from your field and I can only guess what you plan to work on. The maybe humorous answer does not intend to make fun of anything (I hope it's clear, otherwise I'm willing to retract it).