To retrieve files from a GoPro 9 Black on linux, you need to change a default factory setting on the camera. The setting is under Connections -> USB, and you need to change the connection type from “gopro connect” to “MTP”. Once you do that, you can plug your gopro into a USB 3 or USB-c port and transfer files from a mounted file directory. Most modern distros recognize it as a cell-phone device. From there, the files can be found by double clicking the Hero9 item in the file chooser, and then by going to the DCIM folder.