Recently, I need to management my code to use GitLab, but there is one question
What is the difference about installing GitLab between 'my server' and 'GitLab server'?
I guess for security.....
but, GitLab be able to select unpublished my code to another developer.
So I really wonder about this.
If by "GitLab server" you are referring to the repos in gitlab.com/explore, the main difference is that you don't have to install anything: you can create a public or private repo and start pushing there.
If you do install GitLab on your own server, then you can control the access to your own server, but you would need to manage that GitLab instance yourself.