Of course. But I usually have a few open browser windows (each containing 5-10 related tabs). If Gmail was also a window, I would have to press Cmd-` (Mac's shortcut for switching between different windows of a single app) multiple times.

But to be honest that's not how I actually do my stuff. I use QuickSilver[1] and when I want to go to Mail.app, I just press Cmd-Space, then 'M' or 'Ma' (because Mail.app begins with M) and press return. It takes less than 0.4 seconds (QS is wicked fast) and is much more convenient than a tab in my browser. Not to mention that a native app is usually way faster and more responsive (I have a ridiculously slow internet connection).

[1]: http://qsapp.com