Newer Garmin devices use the Media Transfer Protocol (MTP) to connect the device as a "drive" to your computer. (Older devices just looked like removable USB drives (with a drive letter) when plugged into a computer.)
In Windows, you can access the device storage from Windows Explorer but it isn't associated with a drive letter. GRouteLoader looks at drive letters.
Download and install the "dokan" library from https://github.com/dokan-dev/dokany/releases/tag/v1.5.1.1000
(You need the older "dokan1" version since mtpmount-x64.exe hasn't been updated yet to use the latest version (dokan2).)
Create a folder named "mptmount" in the "c:\Program Files" folder and download/copy "mtpmount-x64.exe" to the "mtpmount" folder from https://github.com/hst125fan/mtpmount/releases/tag/19.8.0. Having "mtpmount-x64.exe" in this folder lets GRouteLoader to find the program (and mount/unmount MTP drives itself).