With bonus-point wins over Fiji and Portugal in their first two matches, Wales could qualify for the quarter-finals with a victory over Australia, whose loss to the Fijians last week blew Pool C wide open.
Anything less than a win for Wales, however, and the pool could ultimately end up as a three-way tie at the top with the two spots in the knockout rounds decided by tiebreakers.
"There's definitely desperation for us," Gatland told a news conference in Paris.
"A loss or no points and it could come down to points difference. That's the last position we want to be in.
"When you've worked so hard, why put yourself in that position? Teams that are more desperate can let the pressure get to them. We're desperate for the right reasons."
Gatland has reverted to the starting team that edged Fiji 32-26 in their World Cup opener, recalling key backs Dan Biggar (33), Liam Williams (32), Nick Tompkins (28) and George North (31) after resting them for the laboured 28-8 win over Portugal.
"I think there was an opportunity for players against Portugal but we felt that the team against Fiji did what we asked," he said.
"I don't think we’ve got enough credit for that Fiji game. I was really pleased with that performance. It was the highest ball-in-play time and we were in control for 65 minutes."
Ryan Elias (28) retains the starting spot at hooker in preference to co-captain Dewi Lake (24), who failed to make the bench with Elliot Dee (29) named as the backup rake.
"He was disappointed," Gatland said of Lake. "He hasn't had a lot of rugby."
Prop Henry Thomas (31) comes onto the bench as cover for tighthead Tomas Francis (31) after recovering from a lower leg injury, and looks set to make his World Cup debut at OL Stadium.
Openside flanker Tommy Reffell (24), who has also been struggling with a leg injury and was a late withdrawal from the Portugal match, is replaced by Taine Basham (23) on the bench.
"Against Fiji, we created some nice opportunities. We've done that in both games so far," Gatland added.
"I don't think a lot will change. We've talked a lot about how we'll attack Australia. It'll be one hell of a game with both of us being desperate to progress."