We’re still talking about pre-Internet GDSes. In 80-s new systems like that appeared. So Delta, Trans World ans North West created GDS called Worldspan. Three years before that (in 1987) Lufthansa and Air France implemented Amadeus. Here’s the list of biggest GDSes along with actual travel commerce site that use them:
So as we can see these days it’s different first of all because major GDS systems are open not only to travel agents but also to the end consumer. Web capabilities enrich user experience and GDS functionality allows you to build your itinerary on your own so that you don’t necessarily need travel agent to do it for you.