Oops, my apologies...seems we received a sneaky update of the gmail app without being told.
Same as before, go into settings, choose the account you wish to modify settings for, and scroll to the bottom. There should be an option that reads "Labels to notify", select it. Another menu should pop up for all the folders/labels you wish to receive a notification for. Select the one you wish to edit, and in there now are the options to change ringtone, and whether or not to vibrate.
Geez...would've been nice if we'd been notified of the update...