I'd suggest checking out the starred projects for an overview: https://gitlab.com/explore/projects/starred

Just to pick a few on gitlab.com:

- Gitter: https://gitlab.com/gitlab-org/gitter/webapp

- Inkscape: https://gitlab.com/inkscape

- Mailman: https://gitlab.com/mailman/mailman

- F-Droid: https://gitlab.com/fdroid

- TortoiseGit: https://gitlab.com/tortoisegit/tortoisegit

As for self-managed:

- Debian: https://salsa.debian.org/public

GNOME:

- All projects: https://gitlab.gnome.org/GNOME

- GIMP: https://gitlab.gnome.org/GNOME/gimp

- GNOME Shell: https://gitlab.gnome.org/GNOME/gnome-shell

- GTK: https://gitlab.gnome.org/GNOME/gtk

Freedesktop.org projects:

- D-Bus: https://gitlab.freedesktop.org/dbus

- X.org: https://gitlab.freedesktop.org/xorg

- PulseAudio: https://gitlab.freedesktop.org/pulseaudio

- NetworkManager: https://gitlab.freedesktop.org/NetworkManager

- Wayland and Weston: https://gitlab.freedesktop.org/wayland

- GStreamer: https://gitlab.freedesktop.org/gstreamer

- Mesa 3D Graphics library: https://gitlab.freedesktop.org/mesa

In addition, there are a few large open source projects which are in the process of either assessing or migrating to GitLab:

- KDE: https://gitlab.com/gitlab-org/gitlab-ce/issues/53206

- Glasgow Haskell Compiler: https://gitlab.com/gitlab-org/gitlab-ce/issues/55039