If you cannot find a legit guardring PCell (and there
may be multiple, few or none, depending on how the
foundry likes to roll) then you should refer to the PDK
docs (esp. latchup / ESD sections) for construction
rules and, hopefully, some "cartoons" you can use to
hand-build a guardring segment of your own, and then
"stack 'em up". When I encounter such a need I will
build a square guardring unit (which is probably quite
simple at the bottom of it all, but fancy in the verification
/ identification features) and arrange them into rings.
You may subsequently have to pore over verification
decks to see what the first few tries at DRC throw at
you for errors.