@ooskay I am away from my setup, and I don't even know where a config lives for GPIOnext. You might ask questions about GPIOnext on the github repo. The developer has been very friendly and helpful users in the past.
If you want to go the stripped down route, you can find examples of raw python scripts that setup GPIO pins for input all over the internet. In my opinion, the key to making a script successful is to define OS commands as a function that gets called when the GPIO pin is triggered using EDGE detection method. The script ends in a Pause loop or a while loop so it never terminates.
Then, you just need to launch the scrip at startup. Sorry to be so abstract about it, but it was definitely a learning process and everyone has a different config than you.