Wouldnt it better if your application reacts properly to the back button?
I don't know abaout GMail, but in google analytics, the "Back" support just sucks!
I know its hard, but your users expect the back button to work, its not nice to castrate the users browser!
For Jakob Nielsen, tahts the number 1 mistake (of 1999!), and I think he is right!
http://www.useit.com/alertbox/990530.html