I took the time to figure out what changes needed to be made to play the game, so I figured I would share so you don't have to spend the time of trial and error. The game runs pretty well on its own because it is just a Portal 2 Mod but it requires a little adjustment on the controller configuration to allow you to move about and look around.

• Select 'Play' to open the game on your Steam Deck.
  
• Once the game loads press the 'steam' button to pull up your overlay menu and select Controller settings
  
• Select Controller Settings > Edit Layout > Joysticks
  
• The right joystick will need to be set to 'Joystick Mouse' (I changed the sensitivity to 550 to make it a little more responsive)
  
• The Left Joystick will need to be set to 'Directional Pad' With L-Up being set to W, L-Left = A, L-Down = S, L-Right = D.
  
• And that is it. If I find more adjustments that need to be made I will update the guide. Happy playing everyone.

Enjoy.