It's pretty decent

We are using GitLab for git and it's CI/CD capabilities, and that everything is managed and stored within GitLab. One of the major draws of GitLab is their security scans that are only offered in their Ultimate edition. We are using this (and GitLab) for all our projects, which assists us in surfacing security issues, helping us deploy faster, etc. Everything can be self-hosted and self-contained.

Definitely. However, do note that it is not a catch-all thing (nothing in security ever is), so please review, try it out, and see if it meets your needs.

I think this is an implementation question. CICD are just tools in a toolbox. You configure what you want. Separating code into different projects, running scripts to test, deploy, etc. is all up to the user. It is just a way for you to run code. Dockerise if you want. There are a lot of possibilities, and with GitLab updates coming out every month, they are improving (they are a lot better than they were a few years ago).