The plugin saves additional data about whether commenters and users want to display a gravatar or not (if you select that mode in the settings). These data are deleted when you properly uninstall the plugin.
Apart from that, the plugin only filters data, but does not permanently change them. Especially, if you deactivate the plugin und have gravatars turned on, they will again show up for everybody, even those commenters and users who opted out of displaying gravatars. You do have to change the default gravatar back manually, though.
