On premises offline (Gitlab server without internet) Gitlab DevSecOps

Recent past I have installed and configured Gitlab in on premises server without internet connection (on frame). What I have understood that there is very limited help in Gitlab site as well in Google search. Even I have completed CI CD pipeline for Java and .NET projects and while doing the DevSecOps projects, help from GitLab or Google were close to nil. So, I want to share my experience as well as technical help to readers of this blog.