The countries face a quickfire Millennium Stadium rematch after England triumphed narrowly at Twickenham last weekend, and Gatland is only contemplating one outcome.
"We have spoken as a group about how important it is for us to win on Saturday," said the Wales coach, who has called up James Hook, Gavin Henson, Lloyd Burns and Luke Charteris for their first pre-tournament starts.
"If we are going to be serious contenders in this World Cup, we need to send out a message that we are good enough to beat England at home and do that next week as well (against Argentina).
"I don't want us going into a World Cup with people thinking that Wales are good enough to win games, but for that to be a surprise or an upset.
"We need the win this weekend - it's as simple as that.
"We have the toughest pool at the tournament, and in less than a month's time we will be facing South Africa in our opening match.
"It is vitally important we go into that match with the sound of Welsh fans ringing in our ears, fans who have cheered us on to victories in the final two matches of our warm-up campaign against England and Argentina."
Unlike his England counterpart Martin Johnson, Gatland has resisted the temptation to make wholesale changes, although several switches can be expected against Argentina in nine days' time.
The versatile Hook replaces full-back Morgan Stoddart, who broke his leg at Twickenham and will miss the World Cup, with Stoddart's Scarlets colleague Rhys Priestland continuing in the fly-half role following an accomplished showing last Saturday.
Henson's first start against England since the 2008 RBS 6 Nations Championship sees him taking the number 12 shirt with Jamie Roberts moving to outside centre, while hooker Burns gains a full Test debut and his Newport Gwent Dragons colleague Charteris takes over from Bradley Davies in the second row.
Henson has said he needs to produce something "pretty special" as he pursues a World Cup place after missing out in 2003 and 2007, believing three other centres are currently ahead of him in the pecking order.
Gatland added: "There is a lot of competition in that position.
"Gavin summed it up perfectly himself in that he needs to have a good performance."
Henson apart, most attention among an anticipated 70,000 crowd is likely to focus on Priestland, who is mounting a strong challenge to not only make Gatland's final 30-man World Cup squad, but also start against South Africa in Wellington on September 11.
Priestland moved from full-back to number 10 just before kick-off when Stephen Jones' calf muscle tightened at Twickenham, and he was arguably Wales' best player.
"We were impressed with Rhys and felt he deserved another chance," said Gatland.
"For a young player to be told 10 minutes before kick-off he was moving positions, and to handle the pressure that he did and play as well as he did under that pressure away from home, we were impressed with him.
"We wanted to try to keep continuity from last week and build on it.
"We expect perhaps a lot of similarity from last week (with England) - strong at scrummage time, strong in driving lineouts. It is going to be a tough encounter for us.
"And discipline is absolutely vital. Speaking to the players, a big part of the discipline last week (Wales only conceded three penalties) and how good we were was a reflection of how fit we are at the moment.
"There was clarity and confidence from the players in terms of not making dumb decisions when they are a bit tired. Our discipline was a hugely positive aspect of the performance.
"At times, it has caused us problems in the past. As a target, if we can give away less than 10 penalties in a game, it is hugely positive for us going forward."
Flanker Sam Warburton will again skipper the side in injured hooker Matthew Rees' absence, with former Wales captain Ryan Jones ruled out because of a calf problem.
Jones and Rees apart, the other players Gatland did not consider for selection this weekend were wing Leigh Halfpenny (ankle), fly-half Stephen Jones (calf) and prop Gethin Jenkins (calf).
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