On your Steam Deck

Switch over to desktop mode

Open the Discover app

Look for ProtonUp-Qt app and install it

Once ProtonUp-Qt is installed, open it

On the ProtonUp-Qt window, click on add version

Select Proton-GE as the compatibiltiy tool and latest GE-Proton for Version then click install

Once the installation is finished, I'd recommend restarting your device. 

You should now be able to select GE-Proton in compatibility dropdown in both Desktop and Gaming mode.