Are you using the icon 'Gmail' or the icon 'Mail' ? I know if you have gmail, you are using that one, but I am using that and I am using the 'Mail' app, and if you log your gmail account up the the 'Mail' app, then open that app and on the bottom there are 3 headers: Received, Converstions, Attachments. Go to the Attachments sections and choose the message you want (might need to do a 'Refresh' command). Then click the right arrow and the "download" arrow to the left. That will iniate the download and when it is done, where the down arrow is will become a picture of the paperclip (attachment) icon for most programs. When the papperclip is showing, click that and it will ask where you want to save it.
Enjoy, Hope this helps, and like you, I wish the 'Gmail' app could do this. Someone should mention for this to be added to the update.