Software
Software part of the OpenGimbal project consists of the firmware for C1 and D1 and the Companion web app. The firmware is made to work on its own - after you flash and calibrate the gimbal, you don't need any external devices - you can turn the gimbal on/off and control it entirely using the buttons and joystick on the gimbal itself. Companion app is useful for calibrating the gimbal or controlling it from a browser. It also contains the gimbal driver itself (in /companion/src/scripts/driver directory) that you can use for making your own apps as long as your runtime supports WebUSB.
Development
If you can't use the provided driver, it is easy to control OpenGimbal from your hardware and programming language of choice. Both C1 and D1 use a simple protocol and using it is as easy as sending a few bytes over UART. Learn more here.