Bernie,

Every RTW fare has its own tarrif. 'No backtracking' is among the most common rules, but it is not invariable. Some allow for side trips, either for free or at a surcharge, others don't. Some limit North American stops, others don't. You ge the picture--each carrier pair/group sets their own rules.

The reason you were denied LAX-GIG-LAX was that this violates another common rule--no more than one stopover in the same city in any itinerary. I am pretty sure that this rule does apply to *RTW routings. (Which was why I said earlier that MAN-JNB-MAN would not be allowed in TRESY's itinerary).