The Perpignan-bound star had impressed in the 24-6 defeat of Scotland, but Gatland acknowledged the alternatives at centre just weren't good enough.
But the New Zealander warned 98-cap veteran Stephen Jones he would have no qualms about moving Hook back to stand-off before the end of the tournament.
"We didn't have a lot of choice really," said Gatland. "James had a great game against Scotland at outside-half, but I think he's very aware of the need to make this decision.
"We have trawled Wales in terms of what were the alternatives, in terms of bringing people in.
"But that potentially weakens the squad with calls, continuity and so on. Thus the easiest thing for us was just to put James back in the centre.
"James, in the past, has done a great job for us in the centre. He is familiar playing 13 for us and it gives Stephen a good opportunity against Italy
"Stephen knows he's under a bit of pressure in that position and I think he'll respond to it.
"It doesn't rule out the possibility of playing James back at 10 in the competition, but that's going to depend on the performance of the players against Italy.
"The nice position we are in is there's a lot of competition for places, even though it's a small squad."