GAL was already present in the SBS2000 server I once ran (years ago). What I also can tell you is that GAL isn't your friend when you want to automatize sending and receiving encrypted mail, which requires a feature called: 'extended MAPI' (and CAPI, of course). To my knowledge, Outlook is the only email client that supports extended MAPI, any other client only supports MAPI.
Extended MAPI is no fun to work with...and that is mainly due to very sparse (API) documentation from Microsoft. It feels like you need to visit the blog from a friend of the brother of the hairdresser from the neighbor of one of the developers to get (partial) info. This is in stark contrast with documentation delivered by MS for other MS products.
And with each iteration of Outlook and/or Office/Outlook patch Microsoft manages to break this. Consider this a friendly warning, I sure would have appreciated one when I started.