Google Cloud apt signatures couldn’t be verified

When i run apt update, i get error

W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://packages.cloud.google.com cloud-sdk-jessie InRelease: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 6A030B21BA07F4FB

W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://packages.cloud.google.com google-compute-engine-jessie-stable InRelease: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 6A030B21BA07F4FB

W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://packages.cloud.google.com google-cloud-packages-archive-keyring-jessie InRelease: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 6A030B21BA07F4FB

W: Failed to fetch http://packages.cloud.google.com/apt/dists/cloud-sdk-jessie/InRelease

W: Failed to fetch http://packages.cloud.google.com/apt/dists/google-compute-engine-jessie-stable/InRelease

W: Failed to fetch http://packages.cloud.google.com/apt/dists/google-cloud-packages-archive-keyring-jessie/InRelease

W: Some index files failed to download. They have been ignored, or old ones used instead.

To fix this, run

apt gcloud