In my GitLab repository, I have a group with 20 projects. I want to clone all projects at once. Is that possible?
Not really, unless:
you have a 21st project which references the other 20 as submodules.
(in which case a clone followed by a git submodule update --init would be enough to get all 20 projects cloned and checked out)
or you somehow list the projects you have access (GitLab API for projects), and loop on that result to clone each one (meaning that can be scripted, and then executed as "one" command)