My Gmail drafts are not syncing properly between Apple's Mail.app (on the Mac) and the Mail app on the iPad. What is the proper way to set up drafts so that they sync properly?
First - you'll want to set up two draft messages. One on the iPad and check using a gmail web interface if it's really storing the drafts properly to gmail. The iOS uses exchange active sync, but that shouldn't matter to you - either the iPad is storing it to the server (as viewed through mobile safari or any other web interface to gmail) or it isn't.
Repeat for Mail.app on the OS X. Here, you have more control (and more opportunity to set it up improperly).
The main item that would cause issues is if mail thinks it's not supposed to store drafts on the server. See the Accounts preference for your gmail account.
From there, you'll have to use the help menu to look into how to manually synchronize or run the connection doctor to see if/why mail isn't able to store drafts as directed. The last resort would be to hit up the google support link provided at the bottom of the mail account pane. Hopefully, you can suss it out with these tips.