"It was probably the fact that we'd gone with James last week to cover that position," Gatland said. Hook's versatility had been noted as well.
A nagging injury meant Hook could not play earlier, but that gave Priestland the chance to make the role his own. In a meagre field, he has been a standout performer at No 10.
"It's part of sport. You pick up injuries, you don't dwell on it," Gatland said.
"Having the quality of James Hook coming into the side and someone like Stephen Jones coming on to the bench shows what a good position we're in."
Wales' game plan will not alter because of the change of personnel. They have got better with each game, playing with a heady mix of adventure and pragmatism.
They have a set-piece that holds its own, a defensive system that emphasises line-speed and a willingness to use the ball when space appears.
What they don't have is recent pedigree against their semifinal opponents.
"I've been involved in a squad to play France twice and both times we've lost," captain Sam Warburton said.
"But it's a World Cup and the results have shown so far that anything can happen. That's why this game is wide open and should be a good game to watch."
His battle in the loose against the three-headed threat of Imanol Harinordoquy, Thierry Dusautoir and Julien Bonnaire shapes as pivotal.
Last week Warburton, Dan Lydiate and Toby Faletau coped brilliantly with the vaunted Ireland back row and more of the same will be needed.
"It seems to be a strength at this World Cup, everyone seems to have a pretty handy back row," Warburton said. "Obviously Dusautoir has done a great job for them since he's been captain. He's one of the guys I looked up to when I was a youngster."
There'll be little time for hero worship tomorrow night.
For Wales' young players and old heads alike, it will be the most important, and most probably quickest, 80 minutes of their lives.
Gatland has no doubt they're ready.
"At the moment, there's a huge amount of confidence in this team.
"We've got a really good balance of experience and young players with no fear. You can judge that from training.
" We planned to train for about 50 minutes [on Tuesday] but we shortened it up because the quality and execution of the players showed to me as a coach that we're in the right frame of mind."
Best moment so far
The final 30 minutes of their quarter-final against Ireland. When Keith Earls went in at the corner to tie the game at 10-10 early in the second half, there was a sense Ireland's experienced players were poised to take over the match and grind Wales down. What we got was Wales apeing what Ireland would have loved to do. Halfback Mike Phillips took over the running of the game, they pegged Ireland into corners and every player tackled as if their lives depended on it. Ireland were strangled and conceded a couple of soft tries in the process.
World moment so far
It happened early, which was handy as it gave Wales plenty of time to get overit. After dominating South Africa for long spells, they conceded an uncharacteristically simple try to Bok replacement Francois Hougaard. Itwould not have made it so bad if they hadn't twice butchered opportunities to win the game late. First Rhys Priestland missed about as easy a drop goal as you can get in international rugby, then James Hook missed a more difficult penalty attempt.
