it would appear that unlike the IMAP accounts, you can't choose the drafts to be on the server.
if you really need this function you can switch back to IMAP for gmail (i may be going back myself for speed reasons; the active sync is much slower to grab emails if they aren't already pushed).
the gmail drafts should show up though; the online 'Drafts' folder should be right next to the 'apple mail to do' folder (below the [Gmail] group).... actually i have a thought about that:
I tried to use the 'move' command to move from iPhone drafts to g.apps drafts; no luck!
So.. if you really need it; you'll have to go back to imap.
-awr