how do I enable 180 degree screen rotation on Moto G (3rd gen)?
Portrait is default
Landscape rotates the screen by 90 degrees
but is there a way to enable auto rotate for 180 degrees (upside down/reverse portrait) without root access
Use Rotation - Orientation Manager (7-day trial). It allows you to set the orientation per app or globally. "Reverse Portrait" is what you're looking for in terms of 180 degree.
(Click image to enlarge)
Alternatively, for my Paranoid ROM (AOSP; Android 5.1), using adb I can enter on PC:
adb shell settings put system user_rotation 2
It achieves the same goal.
On further search, I found some notable mentions: