Before you configure the advanced settings you should open your account in Mail at least once. This allows Mail to fetch the folders from the mail server.

In this image you can see that Mail has loaded the folders from my Gmail account.

Now open the Settings application.

From the Mail, Contacts, Calendars tab tap on your Gmail account.

Tap on Advanced in the dialog that appears.

Now we are going to tell Mail to store draft messages in the Gmail drafts folders rather than a folder on the iPad. This will allows you to share drafts among all of your Gmail clients.

Tap on Drafts Mailbox.

In the list of folders that appears tap on the Drafts folder that appears under the [Gmail] folder (1) then tap on the Advanced button (2).

Now we will change the folder that is used for the Sent Mailbox. Tap on Sent Mailbox.

Tap on the Sent Mail folder that appears under the [Gmail] folder (1) then tap on Advanced (2).

There are two ways to handle messages you delete on your iPad in relation to what happens to the messages on Gmail.

Approach 1: When you delete a message on your iPad and it is moved to the Trash folder in Gmail. Gmail then deletes the messages in the Trash at regular intervals.

Approach 2: When you delete a message on your iPad the message is archived in Gmail and moved to a trash folder on your iPad. This approach is useful if you want to be able to quickly move messages you are done with out of your inbox and into the archive. If you really want to trash a messages you can always move it into the Gmail trash folder manually.

If you want to use Approach 1 then you can tap on Deleted Mailbox at this point and select the Trash folder that appears under the [Gmail] folder.

For Approach 2 you can leave the default setting (Trash folder on your iPad).

Regardless of which approach you use you should set Remove to Never. Tap on Remove under Deleted Messages.

Tap on Never (1) then tap on Advanced (2).

Your Advanced settings should now look something like this.