I would like to be able to use Jetpack's Gravatar hovercards. I believe that this requires me to be using Gravatar as my Default Avatar. The setting my question refers to can be found in wp-admin by navigating to Settings/Discussion/Avatars/Default Avatar. Right now I'm using Retro as my "Default Avatar" setting for everyone on my wordpress site, but am thinking about switching to Gravatar.
I want to know: if I switch Gravatar to being the default avatar for my site, what will happen for users that don't have Gravatar accounts? If possible, I'd like to have a Gravatar account be used if they have one, but a Retro avatar be generated if they don't. Is that possible, and if so, how? If it's not possible, then again: what happens if the default is Gravatar but a user doesn't have a Gravatar account?
I'm running a WP site that has the latest WP and Jetpack installed.