It is always GNWL first. But truly speaking...you cannot compare both. CKWL waitlist is confirmed on CK cancelled seats and GNWL is confirmed on GN cancelled/ available seats. If GN seats are available and no GN waitlist/RAC is remaining, then only CK waitlist will get confirmed. BUT this scenario is never possible unless some extra coaches are attached.
Also if premium tatkal is present in that train then all empty PT seats would go to Tatkal (CK) quota and then to GN quota.