In one sentence
A borrower's indemnity covering the title company for the gap between closing and recording, when new liens could theoretically appear.
The longer version
Between signing and the mortgage actually recording, a window exists where an intervening lien could file. The gap indemnity backs the title company for that risk so it can insure the lien as of closing.
It is a routine closing document that lets the title company issue coverage without waiting for the recording to complete.
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