get avatar by email (if not set in gravatar)

I use gravatar to show commenters’ avatars in comments. However, 95% of users don’t use gravatar and 99% or so of them use email account on gmail.com where 90% of them have profile picture set. Is it possible to somehow replace the default gravatar image to user’s google profile avatar? Does anyone know any API to do it? Frankly speaking, I’m looking for a function f(x)=y where x – google email address and y – URL to user’s avatar.

Thanks