I don't know about gmail specifically, but gmail which is part of Google Enterprise solution is "very" profitable as a unit.

You have to look at it this way. All Google's product and services are inter-connected. Unlike tradition software company like MSFT, google doesn't build products for the sole purpose of making profit from start. They make products, and they iterate and years from now if it reaches a tipping point they will start making money off of it. Most of google products are depended on each other's success. So saying things like "Is x product for Google profitable?" is a tricky question.

I am sure they have their separate internal account for each products (or maybe not), but as long as each product drives revenue to its profitable product, but individually not profitable; it makes no difference AFA google is concerned.