Rage 2 finally received a release date the other day, and when it does come out on May 14, it'll land on PC, PlayStation 4, and Xbox One. If, however, you prefer to play on PC and would like to keep all your games on Steam or GOG, it looks like you're out of luck.
As with many modern games, publishers seem keen to release them exclusively within their own launcher or client, and that also looks to be the case with Rage 2. Webstore Fanatical says on its store page, that purchasing the game will give the customer a Bethesda Launcher key.
"Please note that the key provided is a Bethesda.net key and not a Steam Key."
Green Man Gaming also states on their page, that the platform the game will launch on is "Bethesda", which could be easily interpreted as the official Launcher.
The game is not listed on Steam.
Bethesda themselves has yet to comment on the matter.