Ok most likely your unit will only allow files with the names gmapsupp or gmapprom at any one time
I have a Garmin Edge 520, and has a map called gmapbmap.img. Time ago I replaced this file (which Garmin provided) with a .img file I generated with QLandkarte GT, based on several .img tiles based on OpenStreetMap data. I gave that file the same name (gmapbmap.img) so that it was visible on my Edge 520 unit.
I have also a file called gmapbmap.sum (that I didn't touch), plus another pair of files (gmaptz.img and gmaptz.sum) that I believe have to do with time zones but don't know their real function, and I didn't touch either.
If you use Basecamp i suggest downloading this program for your operating system.
JaVaWa GPS-tools | JaVaWa Device Manager
Hook up your unit and close basecamp and run the program, it will scan your device and display your map files, ensure your maps have a basecamp image enabled next to them, close the program and start basecamp, your new map will now be displayed in basecamp.
I've tried JaVaWa Device Manager, and I see my unit and my two maps there:
As you see there is a lock meaning read-only and I can't change the visibility property or anything. Is there anything I can do?
I've also tried and copied the gmapbmap.img file in a USB thumb drive, inside a Garmin folder (as explained in this post). JaVaWa Device Manager sees that drive as a Garmin device and shows my map, and there I can set the property "Visible in BC" to "yes". However, although BaseCamp sees the USB drive as a Garmin device, I can't find that map anywhere in BaseCamp.
I use BaseCamp 4.7.0 for macOS.