How many types of GDS are there?

Related questions

1. What GDS does United Airlines use?
2. Why is GDS important?
3. Is Kayak an OTA?
4. What is Sabre red?
5. What is GDS in travel industry?
6. What is Sabre GDS?
7. What is Travelport ViewTrip?
8. Who are the GDS?
9. How much does it cost to use Sabre?
10. What is the most used GDS system?

Tags

1. #worldspan
2. #sabre
3. #important
4. #amadeus
5. #systems
6. #galileo
7. #travelport
8. #three
9. #there
10. #apollo