Guilty Gear X is the 2nd entry in the franchise, and features a much bigger roster than the first one, as well as updated visuals and mechanics. It's sadly one of the hardest Guilty Gear games to get your hands on, as it hasn't received a proper re-release, and the original version can only be found on PS2 & Dreamcast, or as Windows abandonware.
We do hope that Arc System Works decides to show love to GGX at some point, since they've already done the same for the rest of the Guilty Gear games.