In addition to deciding the regional bragging rights ahead of the summer the day also offers the opportunity for players to push the case for a place on Wales' summer tour of South Africa.
Amongst the intriguing contests on Sunday is upcoming Osprey Dan Baker's battle with Taulupe Faletau and Gatland admits he will be taking notes.
"Judgement Day is the first trial ahead of us picking the squad for the trip to South Africa," Gatland said.
"As far as we are concerned in the Wales management it's a chance for players to put a hand up and show why they should be in the squad.
"There's some fantastic duels in prospect with young pretenders like Dan Baker set to be up against Toby Faletau at No.8 and a fit-again Matthew Rees locking horns with Ken Owens at hooker.
"We will be looking to take our strongest available squad to South Africa but eye-catching performances between now and then could be the difference between going and not.
"I'm really excited about Judgement Day because derbies tend to bring the best out of players. They're as close as you can get to an unofficial trial."
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