The teams meet at Twickenham on Saturday in a "cup final" which the New Zealander says will effectively decide the 2016 Six Nations.
An injury-hit Wales team shocked the hosts 28-25 in a Pool A match in September.
"It was a great performance to get out of that group, but we feel we're in much better shape now," said Gatland.
"There's a lot of confidence in the team and we're treating Saturday as a final.
"It's going to be one heck of a game, and to be honest whoever wins on Saturday probably wins the Six Nations."
Both teams are unbeaten going into Saturday's match, with England boasting a 100% record under new coach Eddie Jones in pursuit of their first Grand Slam since 2003.
Wales have beaten Scotland and France after drawing 16-16 with Ireland in their opening game.
Gatland has named an starting XV unchanged from the one that began the 19-10 win over France.
But there are three changes among the replacements as scrum-half Rhys Webb returns to the squad for the first time since September.
Lock Luke Charteris is also on the bench after missing the win over France, while prop Paul James replaces injured loose-head Gethin Jenkins.
Wales' starting XV shows only four changes from the team that won at Twickenham in the World Cup.
On that day, Hallam Amos and Scott Williams were among the backs, while Gethin Jenkins and Tomas Francis were the starting props.
But Wales have fewer injuries to contend with now, and have a much stronger bench.
"We feel like there's a lot of strength in depth, especially with the two locks - and the two young props have continued to get better and better," said Gatland.
"Samson Lee was coming back from injury at the World Cup so we feel he and Rob Evans have done really well.
"Scott Baldwin is getting better and better so, up front, we feel confident and settled.
"And in the backs, we have [centre] Jonathan Davies back with that experience as well and I'm pleased to see [wing] Alex Cuthbert is getting a little bit of his mojo back."
Gatland believes England are "very similar" to the side Wales played in the World Cup but concedes new coach Eddie Jones has "added an edge" to their play.
"The two wings have changed over and the six and seven have changed over, but apart from that it's pretty much the same team that was on show at the World Cup," added Gatland.
"They're a good side. I don't think they are too far away from cracking it.
"They've got a lot of strength in depth and young players who are improving the longer they are in the side."
Wales coach Warren Gatland believes his team are stronger now than when they beat England in the 2015 World Cup.