Yu-Gi-Oh GX is a series taking place several years after the original series, which can be seen in episode one. The shows main character is Jaden/Judai Yuki, a young boy who dreams one day he will be the King of Games.
A different twist in this show that the original didn't have is that the main characters of the show (Jaden, Syrus, Chazz, Alexis, Zane, Bastion and a few others as time goes on) are school kid age like Yugi and Joey were at the time of the original Yu-Gi-Oh, however rather then be normal school students they live a school made by Seto Kaiba for dueling called Duel Academy. Overall this show isn't half bad, but if you want to truly enjoy the show, I suggest you watch the subbed version of it rather than the Dub so that way you can see every episode of the series the way it was meant to be seen. I also recommend the sub since GX was canceled in the US before the end of Season 3 so the surprise Series Finale never aired here. Personally after reading some it I prefer the manga and recommend it over the show as it has a better more original story.
If I had to give a reason to watch it to you over original Yu-Gi-Oh or 5D's I'd have to say you yourself should be a high school kid, when you watch it you can kind of relate from year to year of the show like I did when it first aired. Once again good series but far from being as good as the original Yu-Gi-Oh!