Problems setting up gmail on ipad
At the moment the way iOS is set up is that if you want to send or share something it will only go through the Mail app.  It's a bit of a bummer, but that's how things are at the moment.  There are slight work arounds, so if you try to attach within an app it uses that app, but in all other cases by default it will always use Mail.

In regards to your Gmail not working, try referring to this article to add your Gmail account to mail.  There are some things that have changed recently with Gmail on iOS devices.  It's not longer setup through exchange so it might not work as effectively as you would if you used the Gmail App.  Your mail will not be pushed automatically you will have manually open the Mail app and refresh to receive new mail.  Another bummer.  

I hope this information helps!