I only have time to give a short answer right now, but I hope it helps:
- In short: NO
- But also: YES, after a fashion
There is a one-to-one correspondence between repositories and projects (which would perhaps better be called repositories as well).
One Solution: Gitlab supports the creation of groups of projects/repos, which can be managed as a project consisting of multiple repos.
Git-based/local Options
If you are interested in git-based solutions to including a repository inside of another repository check out my answer here. If you use either the subtree merge method (at least a variant of it that tracks history) or subrepository method in this answer, your subprojects will appear in your master project in Gitlab, but the master project will also track changes in the subprojects.
Alternative Solution: Create a dummy repo that contains all of your desired repos as subrepos. This master repo will then track all subrepo changes. However; there are a few logistical issues, the .git files for the subrepos will not exist on Gitlab, so you might want a dedicated client with these files to pull the master repo from Gitlab (probably one commit at a time, if you want the subrepo histories to match the main repo history) and update the corresponding local subrepos (these could also be stored independently on GitLab).