I have uploaded a short demonstration video that shows the RetroPie GPIO Adapter in action:
The video shows the RetroPie GPIO adapter connected with two SNES controllers within a PetRockBlock PIE case. The current Raspbian distribution is running in the video with a RetroPie Setup Script installation. The controllers are polled by the gamecon GPIO module that can be installed with the setup script. The button is polled with SNESDev, which is also configured with the setup script.