The one thing I most admire about Gitlab is its installation experience. The upgrades went through very smoothly all the time. This time though when I did “apt update” and “apt upgrade” it gave the following error message.
Err:7 https://packages.gitlab.com/gitlab/gitlab-ee/debian stretch InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3F01618A51312F3F
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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/debian stretch 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/debian/dists/stretch/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.
Based on the error and after exploring Gitlab installation page for Debian here, it was clear that the repository for gitlab needed to be updated. Only the step 2(Add the GitLab package repository and install the package) in that guide has to be executed.
Once the execution of the “script.deb.sh” completes, running apt upgrade did the rest. Gitlab is now upgrade to version 12.9.3.