Shout out to @TimAngus whom I believe is the maker of this Garmin mount.
When slicing you need to separete the individual parts and possition them on the print bed. If you will not do this, it iwll not work (I correct the stl file later). You can check the gcode file, where it is oriented correctly.
I have printed this in 0,1 layer height, other heights I have not tried.
You hold the mount to the seat post with zip ties.
There are two versions, for 28,6 mm and 30,2 mm diameter seat post.
The author remixed this model.
Updated to the mount so it is usable for seatpost.