According to the spinoff of the anime series, Yu-Gi-Oh, titled, Yu-Gi-Oh GX, the letters GX stand for for Generation Next.
How the bleeping Hell GX stands for Generation Next, I will never know. Well, its Generation neXt, I guess. Still seems like bull to me.
ayon kay Namahagetecno37 ika-19 ng Hunyo, 2008