What is a "recordIdentifier" with a gnd prefix in the SRU response?

This "recordIdentifier" is the unique GND number. It consists of the namespace "gnd" and the GND number in the format "gnd:gnd118512676".
Explanation: The GND number is entered in MARC field 035 $a, e.g. 035 $a(DE-588)118512676.
For the "recordIdentifier", the ISIL/MARC Organization Code for the GND "DE-588" is resolved as the prefix "gnd". See also German ISIL and Library Codes Agency (available only in German).