I thought this was meant to work too. On Android I see that Garmin Connect is allowed access to "Smart Notifications" (Settings -> Sound and Notifications -> Notification access), and in Connect I have smart notifications enabled for everything. But I only get call/text notifications, nothing else - not WhatsApp and not Threema and not GMail.
In GCM, go toSettings=>Smart Notifications
Click on the 3 dots at the top RHS.
Click on Add
Scroll down to WhatsApp and click on it.
Select Add
I've added GCM, Maps, Skype, Clock etc. to this list and it works fine.
I was surprised that Garmin had not added GCM by default, but their developers work in mysterious ways it seems.