Dear brother, you can find it by read ... read ... read, by reading more articles you can know the gap in these articles, what the authors did not mention in these articles. For that I want to refer you to use any library software such as Endnote, Mendeley . .. ect.
I normally read about 4 to five articles and writing down all the issues that are not known with certainty. I then identify the issue which is common in most articles as the gap that exists in literature
By reading old and current research papers in the field of research. Usually, excellent researches point out the new areas of research in the field. That may be the easiest way of identifying the research gaps in the area of study.
There is no technique for this but you can develope your skills in such matter by reading a lot, thinking deep, analysing results by your self asking your self some questions and trying to solve them. Also interdisciplinary research and cooperative work with many research institutes may be helpful to control the gap size.