Somehow it passed me by that the release date for Guilty Gear Xrd REV 2 on PC was so close to the European (and Western at that!) console release, so used have I got to fighting games being released late on PC
To get Guilty Gear Xrd REV 2 on Steam you're going to have to do a bit of work, but the short version is that you need Guilty Gear Xrd REV, then you need to buy REV 2 as dlc for the game.
The slightly longer version is that you can buy the game (from HERE) then pick whether you want versions of the game(s) with all the character dlc, costume dlc, audio dlc etc, or the cheaper option of the base game(s) with the option to pick up the extra characters as you see fit
There's a lengthy trailer below that goes in to detail on some of the changes for Guilty Gear Xrd REV 2