Google never keeps its apps exclusive to the Android platform. Google is first and foremost a provider of services, such as email, search, maps, and many more. They will expand those services to any platform that has enough users. This is why iOS has Gmail, YouTube, Maps, and Voice Search apps straight from Google: It benefits everyone. However, there is also Windows. Windows Phone 8 is slowly growing, while Windows 8 got its own store with apps for both PCs and tablets.
Clay Bavor, a Google Product Manager, spoke about the possibilities of Windows apps. Here is his statement:
We have no plans to build out Windows apps. We are very careful about where we invest and will go where the users are but they are not on Windows Phone or Windows 8. If that changes, we would invest there, of course
Guess Windows apps are not currently a possibility. As Bavor said, Windows (Phone) 8 has too small of a user base (outside of computers) for them to invest in developing apps. They have separate teams for each OS, so they’d have to hire all new people. It’s understandable, but don’t lose hope. If Windows Phone 8 catches on, Google very well may change its mind.