$ sudo apt update
...
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://packages.gitlab.com/gitlab/gitlab-ee/ubuntu bionic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3F01618A51312F3F
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://packages.gitlab.com/runner/gitlab-runner/ubuntu bionic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3F01618A51312F3F
W: Failed to fetch https://packages.gitlab.com/gitlab/gitlab-ee/ubuntu/dists/bionic/InRelease  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3F01618A51312F3F
W: Failed to fetch https://packages.gitlab.com/runner/gitlab-runner/ubuntu/dists/bionic/InRelease  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3F01618A51312F3F
W: Some index files failed to download. They have been ignored, or old ones used instead.
euikook@argentum:/etc/apt/sources.list.d$ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 3F01618A51312F3F
Executing: /tmp/apt-key-gpghome.SpBMyBEzT0/gpg.1.sh --keyserver keyserver.ubuntu.com --recv-keys 3F01618A51312F3F