The match is a chance for a different group of Wales players to impress following the 20-9 victory over England in Cardiff this past weekend.
Gatland has made 15 changes to his starting line-up and says places in his World Cup squad are very much up for grabs.
While England coach Steve Borthwick named his 33-player squad on Monday, Gatland is still deliberating.
"For me, it’s a good position to be in knowing the team has to perform. There’s a lot at stake and the players are well aware of that. As a result, we’ve mixed the teams up," he told reporters on Thursday.
One of those hoping to catch the eye is centre Joe Roberts, who has been handed a debut in one of the positions Gatlands is unsure of.
"I've been impressed with him for a while with his footwork and carrying," the coach said.
"He’s left-footed which is a point of difference in terms of exit strategy and how you want to manipulate teams.
"I’m interested to see how he goes with Nick (Tompkins). Hopefully (another debutant) Keiron Williams gets a run in the second half (off the bench).
"We are very uncertain about what the combinations will be in the midfield and the make-up of the back-three. There’s opportunities for players to perform and put their hands up."
Gatland believes England will switch strategy at Twickenham to more of a kicking game having been unable to break through the Wales defence last weekend with the ball in hand.
"We tried to change a few things with our kicking strategy and what they’d posted online about wanting to win the kicking battle. We expect them to kick a hell of a lot more than last week," he said.
Gatland also provided an update on several of his injured players as they prepare to host world champions South Africa on August 19th.
"Taulupe (Faletau) won’t be available next week but we are confident he’ll be available moving forward. (Wing Alex) Cuthbert should be fit for next week, Johnny (Williams) will potentially be fit for next weekend.
"(Flyhalf Gareth) Anscombe won’t be fit, but it doesn’t rule him out of the World Cup."
