The gratuity is only automatically added for parties of 6 or more. That automatic gratuity, incidentally, has nothing to do with
DDP. It is included regardless of how you are paying for the meal.
If you are dining as a party of 6 or more, the server will give you a check for the automatic 18% gratuity. If your party is in two rooms, you will have seperate checks because your DDP credits go by rooms, but the automatic gratuity will still be added to both checks because of the total party being 6 or more.
To figure out how much it will be, just multiply the buffet price X number of people dining X 18%. For example, if the total food bill is $200 for a particular meal,
the automatic gratuity will be $36.00.
If your group splits up (at different restaurants, different tables on different ADRs, etc), each party will still get seperate checks, but no gratuity will be added. You will tip whatever you want in that case, with the expectation still being 15-20%, although some tip less for buffets.