How does one ensure their knowledge of a field is complete when doing a literature review I am a new grad student who is both excited and scared as I embark on this transformational journey. One concern that I have is establishing a current snap shot of my field and identifying boundaries that are ripe for further exploration. This seems like an impossible task given the number of journals out there and rapid pace at which new material becomes available. 

I have been reading some posts on this board and it sounds like literature review is only one phase as a graduate student and then you move on. Given that you are on a time constraint one has to move from a reading focus to a doing focus eventually. My concern is I am moving forward with only half the story.

My field is life sciences, specifically gene therapy.
This seems like an impossible task.


You're right, it is impossible.  One can really never hope to gain a "complete" knowledge of a field, or even a subfield.  You just have to do the best you can, in the time you have, to identify the most significant basic work related to your area of interest, and work that specifically addresses the questions you are studying.

Part of your advisor's job is to help with this; they should have a broader knowledge of the field, and be able to help direct your literature search and point out any glaring gaps.  But despite your combined best efforts, there will almost certainly end up being significant work that you overlook, or whose relevance you don't recognize.  That is just part of doing research.  Someday you'll learn about it and wish you had known it earlier, but all you can do is move on from there.