The source code is in the INDI 3rd party library github.com/indilib/indi-3rdparty
There is indi-rpi-gpio, libpigpiod and indi-asi-power which is the version that controls the ASI Air power controller.
The Astroberry relays driver also works fine and is much simpler. The RPi GPIO driver provides Pulse Width Modulation e.g. for controlling LED brightness or motor speed control, and periodic switching e.g. for triggering a DSLR imaging sequence