How do we enter a gross-up on a W-2?

Q: The company offers stock to employees, which becomes taxable at a later date. Employees who terminate during the year are still eligible for the stock, but, since there are no longer wages from which we can withhold, we gross up the FICA taxes. On their W-2s, we make entries in Boxes 1, 3, 4, 5 and 6. Management is questioning why, if we’re only paying the FICA taxes, all of those boxes are increased. Now we’re not so sure if we’ve been doing this right. Who’s correct?