Install Free Maps on Garmin BaseCamp (Openstreetmap) - No Place Like Outside
A quick guide to free OSM maps for Garmin GPS devices. - ScarletFire Cycling
How to download free OSM maps for Garmin Edge - for ANY country - ScarletFire Cycling
opentopomap and GB .img download for Garmin devices - OSM Help
OpenTopoMap Garmin Maps
Garmin/Nüvi series - OpenStreetMap Wiki
garmin.openstreetmap.nl has been broken and unable to download maps for about a month, I finally tried out openmapchest, it great wish I was using it all these years : r/openstreetmap