kubectl delete -f https://raw.githubusercontent.com/open-policy-agent/gatekeeper/master/deploy/gatekeeper.yaml
Make sure that all the CRD were deleted:
kubectl delete crd \
configs.config.gatekeeper.sh \
constraintpodstatuses.status.gatekeeper.sh \
constrainttemplatepodstatuses.status.gatekeeper.sh \
constrainttemplates.templates.gatekeeper.sh