Do not count on getting the Gray Stuff just by asking, even if you are celebrating. Not saying it doesn't happen, but we've been there 3 times and only got the Gray Stuff once...and that was unexpected.
Make sure you get the celebration listed on the reservation. We forgot to put our son in law's birthday on the actual reservation...told the waiter we were celebrating his birthday...it really was his birthday! No gray stuff. I even asked again, specifically, at the end of the meal, and the waiter said "Sorry, that is only for special celebrations like birthdays or anniversaries." When I pointed out we were in fact celebrating his birthday, the waiter just shrugged his shoulders and disappeared. They were not busy that night, we were eating around 7:00 PM, and at least 1/2 the tables in the main ballroom were empty, so it wasn't that he forgot or was too busy. In fact, the servers were standing around chatting with each other much of the time we were there...yet out at the check-in desk, they were turning hordes of people away, claiming "totally booked" They were not even busy. When we ate there in January, the table next to us had 3 little girls who had been to BBB...very obviously dressed for the occasion. One of the moms at the table leaned sideways to talk to their server and ask for the Gray Stuff for the little girls...the waiter explained to her it was for special occasions only. 3 dressed up little girls were not "special" enough that time either.
Now in February, we had a late dinner there...we were seated about 20 minutes before park closed. At the end of the meal, the servers came out with large trays of The Gray Stuff and delivered a plate to every table. We assumed it was because they had made too many (don't think the stuff would keep well...it's an oreo-mousse.)