A Google Maps Help thread reports that Google accidentally removed the send feature to Garmin GPS. Here is an old video that shows how that worked:
The bug was first reported on January 31st and a Googler replied a day or so ago, saying:
I wanted to let you all know that the Maps team is aware of this issue and is working to fix the feature soon. I know this is an important feature for several users and I can assure you that it's not our intention to remove it from Google Maps.
I'll post an udpate here when I hear more news!
It is still not fixed, as far as I can tell. In any event, Google will likely be adding it back soon.
Forum discussion at Google Maps Help.