Thanks for visiting the Community.
I know how convenient it is to be able to add your Nest Protect device to the Google Home app, but this isn't supported. Meanwhile, you can control and access your Nest Protect on the Nest app itself.
We're always looking for ways to improve -- we'll take this as feedback, and we'll gladly pass it along to the team. You can also send feedback using your device by saying, "Hey Google, send feedback." Visit this link for more steps.