Tap Edit at the top of the Mailboxes list. Tap New Mailbox at the bottom right of the list. Make sure you specify you Gmail account as the Mailbox location.
Or you can log into your Gmail account in Safari and add the new folder there. It will sync back to the Mail app.
You can also use the Gmail app on the iPad.
Folders in Gmail are called Labels (if memory serves). They are not quite identical to folders/mailboxes, but the Mail app treats them as such.