I had looked at that briefly in my previous scrollings through the official GM part store: My understanding is that this panel is just attached to the door panel w/ adhesive and uses a replaceable battery and is paired to the car wirelessly in the same manner that the key fobs are paired.
IMO it seems like a waste of money. I cannot fathom a situation where you would need to get into your car where you have neither the key fob on you, nor your phone to issue a remote unlock command via the app. Even if you locked your keys in the car, you can still get inside because the door will unlock (unless you manually turned off the remote keyless entry feature…in which case you can still just use the app).
edit* I find it hilarous that one of their selling points is the “just in case you lock your keys in the car”. Which, again, on any modern GM vehicle with keyless entry, is not possible.