The AirPort Extreme could be used with a Google WiFi network.....IF....you connect the Airport Extreme to the Google router using a permanent wired Ethernet cable connection.
For most users, there would be little to gain with this type of setup, unless you plan to make a long Ethernet cable run to an isolated area where the AirPort Extreme could be used to provide more WiFi coverage. Think of the AirPort Extreme as a separate network in this type of setup, since it would not "mesh" with the Google WiFi.
The AirPort Extreme cannot wirelessly "extend" or "repeat" the WiFi signal from a Google....(or other 3rd party router).....so it could not be used as a simple range extender using a wireless connection.