Well, data is hard to come by, if someone uses "gmail" then I assume they are using the webmail interface, but they can be using it through IMAP and a standalone client. Likewise if someone uses "outlook" then it might be the native client, or it could be the outlook HTML interface.
Personally, I don't think I know anyone that *doesn't* use gmail with it's web interface. The exceptions are yahoo and hotmail, also with their web interfaces.
Best data I can find is that gmail, hotmail and yahoo mail each have >300M users . It's harder to measure usage of native email clients.