I don't think GX means much, they got A-compound then came up with Q and G. GX is newer version of G, and CRC won WC with. They are releasing latest version of GX35 next month I heard.
Bingo!
From Kimihiko-Yano
We are testing various compounds. To distinguish them, I gave a name in order of A,B,C... And, because the tire that had been named G was good, I decided to sell it. GX is the latest product that gives the same grip level of G tire but eXtend. - I think he means extended wear characteristics or grip level. Or both!