Why this question is closed?
Well, it says so right in the closing notice:
"There are either too many possible answers, or good answers would be too long for this format. Please add details to narrow the answer set or to isolate an issue that can be answered in a few paragraphs."
You gave neither a level of detail/math at which you would like the possible answer to be, nor did you indicate which parts of GR and string theory are you at all familiar with. A rigorous answer adressing someone without prior in-depth knowledge of both topic would possibly fill some pages (to state it mildly), while there might be many different and inconsistent non-rigorous handwaves people could give you. As it stand, the question reads basically: "Explain string theory to me and how it could produce GR results" People spend entire lecture series on this, so it is definitely not a narrow enough question for our Q&A format.