Learning Amadeus GDS is a great idea, Amadeus is used by many international Airlines and Travel Agents. You can learn Amadeus GDS with Pre GDS basics sessions so that you are Job ready and Business Ready. 

Many online sites are there, but you have to look for one where you can also learn the Pre Basics of GDS before learning the Amadeus GDS so that you can understand the Travel Basics then Amadeus GDS. 

Amadeus GDS commands are easy to practice with Amadeus GDS, you can access Amadeus GDS course by singing up and start learning Amadeus GDS with easy step by step guide.

All GDS systems like Amadeus GDS, Galileo GDS, Sabre GDS, and Worldspan GDS are good as long as you know how to use them, they all work the same and they are worldwide famous. 

All travel agents use at least one GDS system for booking and Reserving the airline seats for their customers. Working as a Travel Agent in the market, you need to work on at least of the GDS, like Amadeus, Galileo, WorldSpan and Sabre.