Double tap on either airpod to trigger Google Assistant. It's designed to work when screen is locked. When screen is unlocked it doesn't always work because Android sends media button event to the current front app and AirpodsForGA won't be able to capture the event.

From version AirpodsForGA 2.0.2:

Improved app stability

All versions
November 29, 2017
November 3, 2017
October 10, 2017