The RetroPie GPIO Adapter is compatible with the Raspberry Pi 2.
If you already checked the connection between each pin of the SNES connectors and the corresponding pin on the RetroPie GPIO Adapter, it might be that there is a confused connection. Just a guess, though.
SNESDev installs two gamepad devices in the operating system and gets their states by polling the controllers. If everything is wired up correctly and you press the direction buttons for the first time after boot up, the -32767 will disappear and be replaced by 0 then. This comes from the current implementation of SNESDev.
Does the the button on the GPIO Adapter work?
You could also post a picture of your wiring here.