What is the scientific term for a “research gap”? After I read about 40 papers in my research topic, I summarized 25 of them in my thesis, explaining methods used and solutions provided in the literature survey and literature-review sections. I found out a gap in the previous researches and would like to point it out in my thesis, in order to explain where my work would fit.

Should I call this section research gap, or there are a better scientific terms usually used?
"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".