Not really, since the salt would need to be publicly known for Gravatar to work (and it would break any backwards compatibility to add it in now). This was a 'social engineering' attack, not a rainbow table lookup – it pieced the name together with common providers to find a matching MD5. Salt would just add a single extra step.
I believe it's exactly the same problem/attack as was brought up about MicroID in the past. The idea of Pavatar is a much better way to do this sort of avatar-finding (though the decentralisation comes with its own problems), since it relies on a public web address instead of a semi-private e-mail address.