W: An error occurred during the signature verification.

W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error

みたいなのが、apt update で出る。

• Google Cloud PackagesのGPGでエラー。2018/04/02 - Qiita
• gcloud - GPG error: http://packages.cloud.google.com/apt EXPKEYSIG 3746C208A7317B0F - Stack Overflow

この辺の対応かな。

curl https://packages.cloud.google.com/apt/doc/apt-key.gpg | sudo apt-key add -

を適当に実行してみた・・・。