Unfortunately, this isn't going to be super-straightforward.
To add support for an IDP beyond what GITKit provides, check out this documentation:
In order to run your own IDP, you could try to build some sort of meta directory via Google Apps (which can be chained to other systems via SAML) or
Windows Azure. You may also want to look at tools from vendors like Ping Identity. I believe this kind of setup is one they pretty commonly support.
thanks,
AD