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
AirpodsForGA 2.0.2
November 29, 2017

Version:2.0.2

Uploaded:November 29, 2017 at 12:38AM UTC

File size:1.19 MB

Downloads:5,195

AirpodsForGA 2.0.1
November 3, 2017

Version:2.0.1

Uploaded:November 3, 2017 at 6:46PM UTC

File size:1.19 MB

Downloads:4,124

AirpodsForGA 1.10.0
October 10, 2017

Version:1.10.0

Uploaded:October 10, 2017 at 5:41AM UTC

File size:1.17 MB

Downloads:275