The GRE key referenced in the request wasn't found (GRE key not found) -- relevant to PMIPv6-based interfaces.
Where you see it
In the Cause IE of a GTPv2-C response (Create Session, Modify Bearer, Delete Session…) on S11, S5 or S8.
What to check
Check the GRE tunnel configuration between the two nodes.
Check which node sent the response (MME, SGW or PGW): the cause belongs to that node's decision.
Got a trace with GTPv2 cause 80 GRE key not found? Drop it in the analyzer: it finds the step that really failed first, which node sent the code and what to ask the other side. Captures, test-system PDFs and vendor exports. 5 free analyses a month, no sign-up.