But having not trained all week, Priestland has been ruled out meaning Hook, who has played just three minutes at fly-half in the competition so far, gets the nod over Wales' record caps holder Stephen Jones.
Hook featured at full-back against South Africa and Samoa but sat out the matches with Namibia and Fiji before coming on at the death against Ireland.
However, he did play at No.10 in Wales' last warm-up game against Argentina in August and three of their RBS 6 Nations matches, including the 28-9 defeat to France.
"Time was against Rhys, but we are happy that we have the talent and ability elsewhere in the squad to cover him," said Gatland, who is looking to guide Wales to their first ever World Cup final.
"Rhys has done everything we have asked of him and more during this World Cup, but, when we have players of the calibre and experience of James Hook and Stephen Jones coming into the squad and starting line-up, there is no sense that Rhys' absence leaves us any weaker.
"We know that the people in Wales and the fans that have travelled here are excited about reaching the semi-finals of the World Cup and we are too, but beating France on Saturday must be and will be our sole focus.
"The time to reflect and to celebrate will come if we win our next two matches, until that happens our focus on the task ahead must be absolute."
With lock Luke Charteris shaking off a shoulder problem, Hook replacing Priestland is Wales' only change from the side that beat Ireland with Jones coming onto the bench.
Charteris, Jamie Roberts, George North and captain Sam Warburton sat out training on Tuesday with minor niggles, but all four have been passed fit to face the French, who have won six of their last seven meetings with Wales.
Wales team to face France:
15. Leigh Halfpenny - Cardiff Blues
14. George North - Scarlets
13. Jonathan Davies - Scarlets
12. Jamie Roberts - Cardiff Blues
11. Shane Williams - Ospreys
10. James Hook - Perpignan
9. Mike Phillips - Bayonne
1. Gethin Jenkins - Cardiff Blues
2. Huw Bennett - Ospreys
3. Adam Jones - Ospreys
4. Luke Charteris - Dragons
5. Alun Wyn Jones - Ospreys
6. Dan Lydiate - Dragons
7. Sam Warburton - Cardiff Blues
8. Toby Faletau - Dragons
16. Lloyd Burns - Dragons
17. Paul James - Ospreys
18. Bradley Davies - Cardiff Blues
19. Ryan Jones - Ospreys
20. Lloyd Williams - Cardiff Blues
21. Stephen Jones - Scarlets
22. Scott Williams - Scarlets