That button mapping in USB Loader GX is actually a setting built into Devolution - USB Loader GX cannot change the controls itself. If you're feeling adventurous you could always open up your controller and physically remove the button. Just make sure you put it in a safe place in case you want to put it back! (I removed the shoulder-button springs from an original classic controller years ago but I somehow lost one spring...)
He already knows about the issue. Supposedly the GameCube was originally going to have analog face buttons much like the PlayStation, but it was dropped from the final console hardware. However, the console can still support commands for these analog face buttons, and Luigi's Mansion being a launch game used these instead of the later-standard code for digital button-presses.
Thanks for your reply!
Actually,I wonder if you know ( or can ask himself ) if Tueidj wants to solve the issue in a next Devolution revision.