How to find a gap in knowledge, for my PhD? I just started my PhD and this is my fourth week. I understand that as a PhD student, my main aim is to find a gap in the knowledge and then trace this gap by using scientific approaches that I might choose later. 

My question is: How can I find this gap?

I can think of two options: 


Reading, which is the ideal option and we all know that. However, it might be frustrating to read a lot because maybe you
will get lost in the final stage due to information overload. 
Just jump to the conclusion of any paper in your field and see if they provide any recommendations about a potential gap and choose it as your hypothesis or the starting point for formulating your question.
Ask the advisor.
If he/she is helpful, follow their advice.
If he/she is not helpful, either 


change the advisor (which is preferred) or else
find an interesting and open problem in your field (in some areas such as mathematics, searching for "open problem" or similar in recent publications, including literature reviews, might help). The problem should be such that that you should be able to solve it within the time allocated for your PhD studies, including writing your dissertation. Then, ask the advisor again whether the problem is suitable for your thesis. How to proceed from there would be a different question.