They don't use a GDS on the front end, but doesn't all that data feed into a GDS somewhere on the backend? I don't know much about Sabre but I know Amadeus reaches pretty far across everything.
Once your travel details are in the GDS then the airline clerks, car rental sales person, hotel clerks, and your travel agent can check out or update your travel info for you just based on the PNR number on your airline ticket for convenience if you're just not into that weird smartphone thing. You can change a bunch of things during the trip like what car you drive and hotel amenities during your stay so after the trip ends we download your trip from Sabre and build reports for your boss or whatever.