We actively try to make improvements so GitLab uses fewer resources, but with the scope of the application it's definitely not easy to make it super "lightweight".
I mean, I don't use GitLab but I didn't really mean it disparagingly - for the OP's purpose, GitLab is massive overkill, and will need a lot more resources than a more targeted solution will.
Many interpretations of light weight though, I would say GitLab is light weight in the sense that it's really easy to get going and define jobs. If you wanna interpret it as a light weight install procedure, then I guess no. Depends on your infrastructure already in place.
In terms of software, particularly installed software like this (rather than say a library you rely on in your own code) "light weight" almost always refers to the resources required to run it - prominently memory and CPU power.