What is the scientific term for a “research gap”?
"Literature review" would be a good term. It refers to the section in a paper where you review the existing literature and show where your new piece of research fits in.

Don't confuse this with a "review article", which summarizes and integrates a large number of previous publications, typically far more publications than you'd review for an original article. A review article typically only does the synthesis, and does not add any original research of its own, although it may well point out gaps in existing research for subsequent investigations.

In you want to draw specific attention to the fact that the question you will investigate has not been covered in previous research, you could call it "open questions".