The problem with these gotek drives is they dont work well as an extra drive. This is why I removed it from my A4000T. In order to use disk images you have to boot this drive with that selector/adf at the root of a properly configured flash disk. The menu it boots into is the only way to assign disks to slots that the buttons on the front match. 000 is the selector, 001 - like 999 is self assigned one by one from images on that flash drive.
You cant simply put in an image and use it or access it directly from the USB flash drive through Workbench as if it was a USB drive on a PC or using a real USB stack on an amiga with a USB card installed.
This is true and a major drawback for the use of Gotek. I wish there was a way to use it the Gotek as a normal floppy drive to be able to access the adf from within WB when needed (non auto run games). This would be great to load adf of applications for instance.
For now, I continue using Diskimage GUI to mount and use adf.