GoPros are compatible with their own webcam desktop utility that can be downloaded from here. The application is still in beta mode for Windows users. However, it still has a pretty stable build on the OS. Mac version of the utility works flawlessly.
Once you have the application downloaded and installed, all you need to do is hook up your Hero9 with a USB-C cable and select the GoPro as your preferred capturing device.
A detailed step-by-step walkthrough of the process can be found at a community post on the GoPro website here.