My last two DONE5 were booked with a TA who is/was a member here. My last one was the ex-SEZ deals and those caused A LOT of headaches for TAs who booked them because BA (who had the fare filed) decided they didn't want to honor them and threatened debit memos. That said, oneworld RTW tickets have been able to be autopriced in Sabre for many years and because of that Sabre gives a guarantee to TAs who use it; therefore, BA's debit memos would have been paid by Sabre. Sabre of course balked at having to pay $4,000-$5,000 per ticket, so it caused some serious headaches to the travelers and the TAs. I personally had zero problems with my ticket, but I know of several others that did. From my understanding the TA I had used lost/was going to lose the ability to issue BA tickets because of it.

So, yes, TAs can use a GDS to issue RTW tickets, but might have switched to using the website to avoid airlines deciding the price is too low post-purchase.