I think the problem is with the latest version of the garmin browser plugin. That is how maps finds out the gps.
The answer probably is on the Garmin web site, Google does not allow routes to be transferred only POIs and address locations. See https://buy.garmin.com/support/searchSupport/case.faces?caseId={f868fd20-12df-11dd-dc9c-000000000000} to verify this and get further information.
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