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I run several travel sites in different markets. If you can provide specific technical questions I'll have a go at answering them, as will others here I'm sure.
I'm not sure that you can simply buy into the GDS systems without being a recognised travel agent. However, there are lots of travel affiliate programs that offer free content for you to add to your site. They provide the booking system and deal with the GDS's so you don't have to worry about it. If you want more than a simple linking site some programs provide an XML interface or Web Services that allow you to book inventory through their system whilst maintaining full control of the process.
Hope this helps