I'm afraid I can't even guess what "gross up" means.
The verb "gross" means:
verb[vn] to earn a particular amount of money before tax has been taken off it: It is one of the biggest grossing movies of all time. The concert grossed a massive £2 million at the box office.
gross sb out (NAmE, informal) to be very unpleasant and make sb feel disgusted disgust: His bad breath really grossed me out.