This happened to me when I downloaded Mountain Lion. It included Mail 6.2. All my other folders, except inboxs, disappeared.The sent mail since the download is appearing.
In Apple Mail I created a Smart Mailbox and set the rules to be "From" Contains: "my email address" and then named the mailbox "Sent". Plus, I clicked to include all emails that are in "Sent".
In Google Mail I labeled all the "Sent" mail to be additionally labelled with the "IMAP Sent Mail". The IMAP folder was auto-created. (I don't know when)
When I went back to my Smart Mail Mailbox named "Sent" all the sent mail from me what there.
When I checked the "Sent" box, I had complete threads of emails that were sent.