Neither G nor Gear Watch has ambient light sensors, meaning you have to manually change the brightness like a caveman. Display Brightness for Wear can fake auto-brightness by monitoring sunrise and sunset times in your location and adjusting brightness accordingly. It also knows if you’re driving, in which case it cranks the brightness as well. There are no settings to mess with, but it works as advertised.
Display Brightness for Wear (Free)
Better Wear Face