I've never heard an email sent via Gmail referred to as a "Gmail" before.
It doesn't always require the service have 100% market dominance either, just that it be popular enough people hear others use it and commonly used in exclusivity by the user.
No one says they're going to "Kleenex their nose" or "aspirin their headache" so why would they "gmail their friend"?
You've at least heard someone say "let me Google that" or "that's photoshopped" right?