As a general lesson, what we learnt is these are not bots. These are real humans working in some poor country manually creating accounts (always gmail accounts) and pasting all sorts of random text. Some of these people even setup 2FA and open issues with junk text, it's amazing. Unfortunately, GitLab from what I can tell cannot make issues read-only to non project members (i.e I only want project members to open issues, others can just read and watch issues).
Currently, our forum spam (https://forum.cloudron.io) is way more than GitLab spam. On the forum, we even have Captcha enabled (something we despise) but even that doesn't help when there are real humans at work.