Gitlab blame functionality not working: 502 error

Solution 1:

Note that the GitLab Web UI is no longer the only possibility to trigger a blame.

There is now (August 2019, GitLab 12.2) a GitLab blame API (from Oleg Zubchenko)

In GitLab 12.2, the new Blame API allows this information to be retrieved directly from GitLab, without needing to checkout the repository.
This is helpful for scripting and automation, based on the people who’ve recently changed the file.

See documentation and issue.

I suspect this will have the same limits you have seen with your large repository though.

You can see "git blame" with GitLab 14.7 (January 2022), but only for code search result, in the non-free tiers:

Go to Git blame from code search results

Users often want to understand more about code search results, such as when was a file changed or by whom.

With GitLab 14.7, users can easily answer these questions with fewer clicks by using the View blame link in Global Search results.
This change adds an additional link next to lines of code when hovering over results from a code search.

See Documentation and Issue.