IATA and places like ASTA and such have on line study courses for GDS systems. Some are expensive, some are cheap, but you get what you pay for. Some are geared to all systems some are based on just one or two systems and gives you a good taste and teaching of the systems.
Tourism Colleges can get costly, I'd try online or self study first. If you're a TA then you should have access or association with something that has training like ASTA or IATA.