I have done this so can confirm it works.
I read somewhere the Bosch Intuvia USB outlet is relatively weedy compared to some, but there's certainly enough poke for a Garmin.
One problem may be some Garmins don't like being used while plugged into a charging source.
A powerbank is just as easy in most applications.
My Garmin Touring will nearly go all day on its own battery, so it only needs an hour at the cafe stop on a powerbank to get over the line.