Double-clicking the gmapi file should cause MapManger to open and ask if you want to install the map. Does this happen? IF not, then there is either something wrong with the map or with your installation of Garmin's software. Also, if it appears to be a
folder instead of a file, then there's a problem with the Garmin software. If this is the case, re-install Garmin's software.
If all of this seems to be OK, then can you see the map in Basecamp? If you can, then you should be able to install the map on the GPS. I created my own MapInstall instructions for my maps. My maps are rather unique, but you may find the instructions helpful. See the section "send the map to the GPS" starting on page four here and see if it helps:
https://boydsmaps.com/docs/Boyds-Map-of-NJ-2018-for-Mac.pdfNow there are also issues with Garmin's newest version of MapInstall but it's not clear to me whether they affect the Mac or just Windows. However, you don't seem to be getting far enough for that to be the problem.