My general experience with Amazon has been that they provide warranty support no matter where the purchase was delivered. I live in Latvia and often order stuff from Amazon UK, Germany or France. In the past I had issues with a Logitech mouse and had no problem getting warranty support (twice) from Amazon UK. They even let me only send them the defective one AFTER the new one had arrived. Honestly, Amazons customer suport has been the best I've ever had to deal with, they even refunded me shipping costs when I returned the defective mouse.
You could also contact their support and ask, to be sure.
EDIT: I'd also like to add that in some cases manufacturers themselves provide warranty support even when not purchased directly from them. That depends on the company, so if you're really worried you could also contact the support for the specific manufacturer of the GPU you are buying (ASUS, MSI or whoever makes the specific model)
