Once you have a google space channel you'd like these notifications to be sent to edit /etc/sysconfig/google-chat-notifier and set the NOTIFIER_CHAT_URL environment variable to the webhook for the channel.

For any service that you'd like to have a notification about modify the service definition and add

[Unit]
...
OnFailure=google-chat-notifier@%n.service

This will send a failure message when if service fails.