The key seems to match the one listed in the docs. Tried re-importing the key and setting up the repo via the downloadable .sh script again, same results.
Noticed that there were 2 topics recently related to key expiry for other packages, but I couldnt find any info on the GitLab Runner package.
The key that expired was the one used to sign the repo metadata on packages.gitlab.com so it impacts runner as well. This is different than the key used to sign the runner packages themselves, which is what the runner docs were instructing you to check.
I followed the instructions in the updated docs regarding key removal, and used the rpm .sh script to set up the repo again which included adding the new GPG keys for repo metadata signing. After that, runner packages are successfully installed.