Do this:
curl -s https://packages.gitlab.com/gpg.key | apt-key add -
apt-get update
that worked for me on Debian 10 and Debian 11, so should work for you assuming you use Debian/Ubuntu.
This post should also provide extra info: Omnibus-gitlab packages gpg key expired - #6 by twk3 which provides this Gitlab doc: Cryptographic details related to `omnibus-gitlab` packages | GitLab
And this one: GitLab GPG expired today (#6701) · Issues · GitLab.org / omnibus-gitlab · GitLab of which worked for me using the two commands I gave above.