Cullen was forced to withdraw from the 25-man panel- currently based at Dublin's City West hotel - with a back injury, and with both Ulster second rows Gary Longwell (neck) and Matt McCullough (foot) unavailable, 6ft 7in Gissing has been drafted in.
Auckland-born, Gissing has been in solid form of late, appearing in 13 of Leinster's 15 Celtic League games, and pocketing two tries, including a 34th-minute opener against the Dragons.
He also scored Leinster's twelfth as a substitute in their landslide 13-try 92-17 Heineken Cup Pool 2 trouncing of Bourgoin before Christmas.
Meanwhile, Munster flanker Alan Quinlan is recovering from the broken nose he sustained in his side's 32-17 loss at the Scarlets on Saturday night. The 30-year-old has retained his place in Eddie O'Sullivan's squad.
The starting line-up and substitutes for Sunday's RBS 6 Nations opener in Italy, will be announced on Wednesday afternoon.