If you have Gmail set up on your iPad, you can get to your Sent e-mail box from within the Mail app itself.
With the iPad in landscape mode, in the upper leftmost of your folder list should be a button. Keep pressing it until you get to the main Mail folder screen/list. It should be split into two categories: "Inboxes" and "Accounts."
If you click on the "Gmail Account" (under the Accounts label), you'll be taken to all the folders you've created for/under Gmail. One of the first few folders will be the Sent mail folder.
Hope that helps.
Marilyn