There is tools what allow to play now non Steam games on Steam Deck console and it is easy.
For example you have games library in Epic Games store or in GOG.com and you want to play them on Steam Deck console, what to do?
1. Go to Desktop view
Steam Menu > Power > Switch to Desktop

2. Open web browser, Firefox is installed by default
3. Install NonSteamLaunchers
for that do next:
a) Go to link: github.com/moraroy/NonSteamLaunchers-On-Steam-Deck
b) Scroll down little bit and click on big button “Download”:

c) Download file to your Steam Deck desktop.
d) Run downloaded file NonSteamLaunchers.desktop.download

e) Choose launchers what you want to add to Steam and click “OK” button
Different environments what you can choose:
- Epic Games
- Ubisoft Connect
- EA App
- Gog Galaxy
- Battle.net
- Amazon games
- Xbox Game pass
- Playstation Plus
- Geforce Now

4. Go back to Steam Gaming mode view – on desktop there is icon for this “Return to Gaming Mode”

5. Enjoy!

You can now in Game Mode start different launchers and install games there. Remember that launchers and games what are installed from others launchers will be in library under “Non Steam” tab (if not used or played lately then on lately used grid)

