The difference is in marketing. Gmail is a service with its own official app and so on. Matrix is a protcol, you can't market a protocol to regular people. Other than techies, nobody cares how their chat works as long it works so if you want to gain traction you market Element as the new hot messaging app and not Matrix the protocol.

Isn't it pretty well understood that Gmail is an email service though?

I don't think that's a good example since Gmail is an email provider which can be used with any email client. Element is a Matrix client that can connect to a matrix provider.