I was able to install Gmail afterwards and also use it (send, receive, calendar). I only tested it with a GMail email account.
Otherwise, I have installed Outlook, which also works.

However, you should install the GMS first. I have not tested it otherwise (without GMS).

There will certainly be an e-mail client installed, but not GMail. To what extent GMail supports POP3 or IMAP, you would have to check with GMail.
You can probably install some Google Apps with a lot of manual work and technical know-how and get them to work, but it should never work as well as on a mobile phone with Google Mobile Services.

I think so. You can still download the Google Apps somehow. They are just not preinstalled.