Não foi possível verificar as seguintes assinaturas porque a chave pública não está disponível: NO_PUBKEY 6A030B21BA07F4FB

Meu ambiente:

# uname -a
Linux app11 4.9.0-5-amd64 #1 SMP Debian 4.9.65-3+deb9u2 (2018-01-04) x86_64 GNU/Linux
# 
# cat /etc/*release
PRETTY_NAME="Debian GNU/Linux 9 (stretch)"
NAME="Debian GNU/Linux"
VERSION_ID="9"
VERSION="9 (stretch)"
ID=debian
HOME_URL="https://www.debian.org/"
SUPPORT_URL="https://www.debian.org/support"
BUG_REPORT_URL="https://bugs.debian.org/"
# 

ao tentar executar apt-get update, recebo vários erros:

# apt-get update
Ign:1 http://deb.debian.org/debian stretch InRelease
Hit:2 http://security.debian.org stretch/updates InRelease
Hit:3 http://deb.debian.org/debian stretch-updates InRelease             
Hit:4 http://deb.debian.org/debian stretch-backports InRelease           
Hit:5 http://deb.debian.org/debian stretch Release 
Get:6 http://packages.cloud.google.com/apt cloud-sdk-stretch InRelease [6,377 B]
Ign:7 https://artifacts.elastic.co/packages/6.x/apt stable InRelease
Hit:8 https://artifacts.elastic.co/packages/6.x/apt stable Release
Get:9 http://packages.cloud.google.com/apt google-compute-engine-stretch-stable InRelease [3,843 B]
Get:10 http://packages.cloud.google.com/apt google-cloud-packages-archive-keyring-stretch InRelease [3,876 B]
Hit:11 https://download.docker.com/linux/debian stretch InRelease
Err:6 http://packages.cloud.google.com/apt cloud-sdk-stretch InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6A030B21BA07F4FB
Err:9 http://packages.cloud.google.com/apt google-compute-engine-stretch-stable InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6A030B21BA07F4FB
Err:10 http://packages.cloud.google.com/apt google-cloud-packages-archive-keyring-stretch InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6A030B21BA07F4FB
Fetched 6,377 B in 0s (7,132 B/s)
Reading package lists... Done
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/apt cloud-sdk-stretch 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/apt google-compute-engine-stretch-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/apt google-cloud-packages-archive-keyring-stretch 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-stretch/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/google-compute-engine-stretch-stable/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/google-cloud-packages-archive-keyring-stretch/InRelease  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6A030B21BA07F4FB
W: Some index files failed to download. They have been ignored, or old ones used instead.
# 

Por favor informar.

via apt-key:

# curl -s https://packages.cloud.google.com/apt/doc/apt-key.gpg | apt-key add -
OK
# apt-get update
Ign:1 http://deb.debian.org/debian stretch InRelease
Hit:2 http://security.debian.org stretch/updates InRelease
Hit:3 http://deb.debian.org/debian stretch-updates InRelease
Hit:4 http://deb.debian.org/debian stretch-backports InRelease           
Hit:5 http://deb.debian.org/debian stretch Release                       
Get:6 http://packages.cloud.google.com/apt cloud-sdk-stretch InRelease [6,377 B]
Get:7 http://packages.cloud.google.com/apt google-compute-engine-stretch-stable InRelease [3,843 B]
Ign:8 https://artifacts.elastic.co/packages/6.x/apt stable InRelease
Get:9 http://packages.cloud.google.com/apt google-cloud-packages-archive-keyring-stretch InRelease [3,876 B]
Hit:10 https://artifacts.elastic.co/packages/6.x/apt stable Release
Hit:12 https://download.docker.com/linux/debian stretch InRelease
Get:13 http://packages.cloud.google.com/apt cloud-sdk-stretch/main amd64 Packages [32.5 kB]
Get:14 http://packages.cloud.google.com/apt google-cloud-packages-archive-keyring-stretch/main amd64 Packages [380 B]
Fetched 32.9 kB in 0s (37.8 kB/s)                           
Reading package lists... Done
# 

Eu tive um problema semelhante, mas nada funcionou para mim, exceto isso

sudo launchpad-getkeys