At the beginning of April, GitLab updated their repository signing key. If you try to update, you will see an error during 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: 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: Some index files failed to download. They have been ignored, or old ones used instead.

The easiest fix appears to be simply to update the signing key:

curl --silent https://packages.gitlab.com/gpg.key | sudo apt-key add -

The trailing hyphen is part of the command; don’t forget it.