That would be a problem with the Google Drive app. They probably forgot, or chose not to support the .pages (and other iWorks) extension(s). I uploaded a Pages document to my Google Drive, and the message I get when selecting it is that the file type is not supported.
Even though the file would not display, it was still possible to tap the share icon and use Open In to copy it back to Pages.
You should still be able to tap the (+) option in Pages' Documents view, then Copy from WebDAV. Use the same address and account login you used to export the document. I can not confirm this, because I am unwilling to get a Dav-Pocket account. I already have more than sufficient cloud accounts to track.
Two other things I'll note.
One: GoodReader, a very handy app, has added Google Drive as one of the servers you can upload/download files with. This app is primarily a PDF viewer and annotator, but it also has decent file management, useful tools, and can connect to a wide variety of cloud and computer services. Very handy, and the method I used to get my document to Google Drive for testing. In iOS 6 Pages allows you to open documents in other apps. Google Drive is not shown, but GoodReader is (as are the Box and DropBox apps).
Two: Box.com which I mentioned earlier does preview iWorks files in the iPad app (not on the website). Unless you have a strong reason for using Google Drive for this purpose, you may want to look at it. iWorks files also display in the DropBox app.
In summary, Google Drive isn't a good choice for use with the iWorks apps at this point. It may improve in the future, but for now both DropBox and Box are superior, with the edge to Box.