Same Wiki for several projects on Gitlab

Question

I'm using Gitlab and I've several projects.

project1;
project2;
project3;

Is it possible to have a common Wiki for these projects?

Answer 1

Every wiki is linked to a project, but there is a workaround:

Go to Settings → Services → External Wiki for your 2nd and 3rd projects and set External wiki URL to the URL of your first project wiki:

Or you can host your own wiki, for example, gollum on your server for the same purpose.

Answer 2

Is it possible to have a common Wiki for these projects?

Yes: (October 2020, 4 years later)
GitLab 13.5 proposes:

Group wikis

For many teams, using GitLab wikis for planning and documentation is a critical part of their workflow. Wikis are so popular that they get over a million views each month on GitLab.com. Despite this popularity, teams have struggled with the limitation that wikis were only available at the project level.
Teams working on multiple projects needed to create separate wikis for each repository, leading to a fragmented experience.

In Gitlab 13.5, we are so excited to bring you group wikis!

With 680 upvotes this was the most upvoted feature in the entire GitLab backlog. While highly requested, making a large project-only feature like wikis available at the group level has been a non-trivial operation. We’ve worked tirelessly over the past year to make it happen and now we can’t wait to get it in your hands and hear your feedback.

Group-level wikis open up tons of possibilities to keep your information at a higher level and accessible to a broader set of people. A few examples of what you can put in your group wikis include team-specific information, coding style guides, and designs for your brand or your company.

We know a lot of folks have been looking forward to this feature and shared their input pre-release. We hope all of you will continue to weigh in now that group wikis are available and we’ve opened up a dedicated issue for your feedback.

See Documentation and Issue.

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See GitLab 16.2 (July 2023) adds (for premium or ultimate only):

Group-level wiki now available in Advanced Search

With this release, we’ve extended Advanced Search to include group-level wikis. Users will now be able to find content in these wikis more easily and quickly than before.

See Documentation and Issue.