You can add it as "Non-Steam" game. Click on the Games menu, and select "Add a Non-Steam game to My Library". Select GTAVLauncher.exe as the executable. To get it to work, you'll need to adjust the Windows file properties of GTAVLauncher. Navigate to it's location, right-click on it and click Properties. Click on the Compatibility tab, and select Run As Administrator. Click OK. Try running it through Steam.
Here's a thread with more info:
https://support.rockstargames.com/hc...nging-language