I have an older Garmin eTrex Vista and don't need any special software. It uses a micro-SD card for the maps which I can access either through a USB cable to the GPS and mounting the card as a removable drive or directly inserting the card into the card reader slot on my computer. Then it's just a simple matter of copying the map file from my computer to the GPS.

For maps I use the free "OpenStreetMap" files:

https://wiki.openstreetmap.org/wiki/...armin/Download