"If you host GitLab yourself, IPv6 is supported as long as your underlying provider can support it. GitLab.com on the other hand does not support IPv6 at the moment due to limited provider support."
Well, there is an issue for that[0]. Also, as of today[1], all shared Runners on GitLab.com are each on their own VM inside Docker containers, so that message does not apply for sure.
"If you host GitLab yourself, IPv6 is supported as long as your underlying provider can support it. GitLab.com on the other hand does not support IPv6 at the moment due to limited provider support."