@y-ello said in How to add a GPIO button to a Retropie (Rpi 3B+) build?:

Can I use a GPIO wired push button for exit emulator? how?

Assuming you have a the button wired to a GPIO (there's tons of tutorials and documents on using a RPI with a button), you can use something like GPIONext to run a script whenever you press the button.
RetroArch has a network control interface, to which you can send commands from the script you attach to the button press (pause RA, open the menu, exit, etc.) - see more details here. Since most emulators in RetroPie are just libretro cores and run within RetroArch, it might be enough to cover most emulators, but you can also simulate a keyboard with GPIONext and simulate a key press for the button push (i.e. for opening the MAME menu).

Thank you very much to you both sorted

I put an exit game button on Gpio in my atari console build by simply installing Adafruit’s retrogame application on my Pi, which makes the gpio able to take keyboard inputs, then I just set one of the pins to activate the escape key.