Since GUC priority up to T-24 is basically by time of request (Status is technically first), it can somewhat depend on when you applied the GUC.
By the time you obtain your boarding pass, GUCs clear at normal complimentary standard hierarchy.
In my experience, not with JNB, but a similarly long route: LAX-SYD for the past 6-7 years, we (Mrs BHammy also DM) have had good luck (80%) GUC clearance, even with 100% waitlisted at booking.
The earlier the booking, and GUC application, the more likely we get upgraded before the gate.
Last year was newer config with suites (less D1 seats). Booked in late March, I think, for October over, Nov back. Cleared on the way home just 5 days later. But didn't clear for outbound until 3 weeks out.
We always go a day early, in case we don't get upgraded, because the flight over is a night time flight after Xcon from chatty-ATL-LAX, so it's actually easy to sleep on the way over even in coach that we've done a couple times.
(we move out of C+ as it's always full, in favor of a set of seats near the back of preferred seating. We usually score a row of 3 to ourselves that way)
On the return, it's a morning departure, but flies thru the night, so not much sleep, then the Xcon back home is brutal when the total travel day is 42+ hours (starting from NZ)
Oh, and day of travel for the LAX-SYD route is on the realm of ridiculous...at 9 mos out, they are like $7k (D1), then they slowly ramp up to day of departure that I think I saw last year at $12k
D1 for $3k is a downright bargain for Delta's longest flight, IMO.