There are three ways to connect Google with smartnest:
You don't have a Google Home device but you want to control your devices using Google Assistant you can configure the IFTTT integration and activate Google Assistant channel as a trigger and Webhooks channel as an action:
Don't forget to Add your Webhooks key to your Smartnest Account
You can go to your Account Settings in Smartnest web application and click on the Activate button in the Google Home section, your e-mail will be added to the users with access to the Action and after some hours you will be able to see it in the services on your Google Home App and link your devices.
Go to Google actions console and create a new Project with the following information:
At the end make sure testing on Device is Enabled and then you will be ready to discover your devices following the second method