With the earlier Nikon cameras like my D5100 the exposures are limited to 30s with gphoto2. The DSUSB allows separate shutter control for longer exposures. I can take exposures over 30s with my wife's D750 but not the D5100. Even in windows the DSUSB was necessary for long exposures with the D5100.
I've attached a photo. The light colored cable is part of DSUSB and plugs into a usb port. There is a separate shutter release cable that connects DSUSB to camera. Also a data transfer cable is needed for gphoto2. Without DSUSB only 30s exposures can be taken with D5100.
Ok I committed preliminary support for DSUSB, please test tomorrow. When you connect, put DSUSB in the port and try. There is a good chance nothing would work, so have TeamViewer ready as well.