DL Basic Economy fares are probably the most restrictive for elites. You get elite luggage benefit, but that is about it. There's no pre-assigned seating and no comp upgrades to either DL Comfort+ (DL equivalent of UA Economy Plus) or domestic FC. They also earn no FF miles or elite qualifying miles. They are actually changeable for a fee ($99 domestic and $199 international) unlike some other carriers Basic Economy fares which cannot be changed and have no credit remaining if canceled (outside of significant schedule changes or airline delays/cancellations where you can refund them). If you buy up to a regular/main cabin non-refundable economy ticket with US origin, all the US carriers currently allow changes with no fees (you will just pay fare differences) and you can get a flight credit for full value of ticket if you cancel.

It does not appear that UA even has any Basic Economy fare filings to South Africa right now, so they won't even be a concern for OP. The only choices to South Africa are Economy fares (which can be changed with no fee or cancelled for future flight credit, but not refunded), and refundable Economy fares which can be refunded back to original form of payment if you decided to cancel. As a Gold, OP would have comp access to Preferred Economy seats and Economy Plus, but not Premium Economy seats (which are larger seats with more recline).