As per the 4.10 release notes [1], OpenShift GitOps 1.5 and 1.6 are supported from OCP 4.8 to 4.10:

But in the OCP 4.9 and older documentation, that information is missing (not only the "Compatibility and support matrix" table, but also the release notes of all the new OpenShift GitOps releases supported in each OCP release).

[1] https://docs.openshift.com/container-platform/4.10/cicd/gitops/gitops-release-notes.html#compatibility-and-support-matrix
[2] https://docs.openshift.com/container-platform/4.9/cicd/gitops/gitops-release-notes.html#compatibility-and-support-matrix
[3] https://docs.openshift.com/container-platform/4.8/cicd/gitops/gitops-release-notes.html#compatibility-and-support-matrix