How to install Proton GE on your Steam Deck
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.