As a waitress, technically speaking, you're paid hourly and "gratuity" is just that --- gratuity. It's something that's given at the end of the meal (or night, depending) as a sign of gratitude from the people you were serving.

With that said, the situation sucks because of his friends not only being short on cash for THEIR bill, but the birthday boy was the one who got drunk enough to hand out $25 which would (and should) clearly be taken as tip money. As a server, I would have accepted it as tip money, but my job and integrity is important so I would never cause drama and run after them defending my tip; I'd just give it back and be disappointed.