Michael Gratton commented:
Hey Andrea, yep, I started a thread about it on d-d-l: https://mail.gnome.org/archives/desktop-devel-list/2019-February/msg00001.html
Consensus seems to be that developers find it useful to work around GitLab search limitations, but no one seems to have a positive story about it bringing in new contributors (which apparently was the reason the mirror was created in the first place).
Others also suggested there that as you say, having some way of pulling PRs over to GitLab may well help, but I guess that remains to be seen in practice, I guess?