How can I specify the Sent folder for Gmail on iPad?
If you're talking about viewing emails in Sent folder, that is turned on in the Gmail settings in a regular browser, preferably not on a mobile device, then "Labels" tab (remember folders are "labels" in Gmail). You turn on/off what imap folders you see on your devices in there.

Another way to view the imap folders is to not use Apple's Mail app, and use "mail.google.com" in Safari.  This has way more options, and one of them, clicking on the 3 bars icon above the first mail, will show you all your folder/labels.