Gatland, who will name his final 30-man squad for the tournament on Monday saw his side produce a clinical display to brush aside the Pumas, who finished third back in the 2007 tournament.
And he believes Wales will head into their opening game against South Africa on September 11 full of confidence.
“We’re pleased with the result, we made a couple of changes and it was tough for the guys coming in,” said Gatland.
“We got the win relatively comfortably and can take a lot from our three performances over the past weeks.
“We’re in good shape physically and have won five of our last seven games. We can take a lot confidence on that plane on August 30.
“It’s the best start Wales have had going into a World Cup which gives us momentum and confidence – we have to believe.”
The visitors had started strongly in Cardiff, but as they did against England seven days before, a strong Welsh defence soaked up the pressure before turning the screw in attack.
Gatland was keen to praise Argentina but was delighted with his side’s clinical finishing display.
“It’s easy for people to underestimate Argentina. We took a bit of time to settle, but we then took our opportunities very well,” he added.
“The second try especially was very clinical, and it is about taking those opportunities when they are there.
“Argentina are quite difficult to pay against at the breakdown, they make it difficult for you get quick ball, but we will take confidence from our last three performances.”
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