SCM Manager (https://www.scm-manager.org/) is a very easy solution to share your Git repositories in a local Windows network; it offers a standalone solution to install and make it work on top of Apache Web Server directly in Windows. Although it is built in Java, you can make it work even in Linux or Mac.

It can manage Subversion, Git, and Mercurial repositories, allowing you to define users, groups, and so on; it has a good list of plugins too, for other version control systems and other development related tools like Jenkins, Bamboo, and so on. There's also a Gravatar plugin and an Active Directory one, to let you and your colleagues use default domain credentials to access your internal repositories.

I have been using this solution for about two years without a hitch, with the exception of some configuration related annoyances during updates, due to my custom path personalization: