how to create custom login page for gmail (google business apps)

You can't. This would create a Man-In-The-Middle attack against the Google services because you would be able to easily know the user's credentials since they would need to enter them into your page and you would pass them on to Google. In fact, if your users were silly enough to blindly enter their Google passwords into a non-Google site, they should be punched in the throat for exposing your business to a risk like that.

You can create an app or a web site that can access a user's mailbox (with their explicit permission) but you do not handle the authentication, it is federated through Google's servers. I wrote an article on accessing Drive from WPF which you can review if you are interested. The principles apply to most any platform you use to access Google services. However, you would basically be creating an inferior copy of the GMail client.