If you're using the Mail app, you can't have it both ways. Either the mail is downloaded for you to see or it isn't downloaded and you can't see it. If you don't want the downloading, delete the account.
I don't know if the iPad Safari browser can be used to access gmail. I can't use it for MobileMe but I do use it for Verizon mail and there's no downloading. Try that and see if it solves your problem.
By the way, you might also configure the iPad for an automatic wipe after ten consecutive unsuccessful login attempts.
There's a lot of good information in the User Guide here:
http://support.apple.com/manuals/#ipad
Also, the (free) User Guide from iBooks is even more extensive and a worthwhile download.