The 28-13 success followed last weekend's triumph against Six Nations champions England, meaning Wales have won five of their last seven Test matches.
Tries from number eight Andy Powell, lock Alun-Wyn Jones and wing George North, plus 13 points from the boot of fly-half James Hook, completed a satisfactory month's World Cup preparation work for Wales.
"We made nine changes for this game, so it was difficult on the guys coming in," said Gatland.
"And it's easy for people to underestimate Argentina. We took a bit of time to settle, but we then took our opportunities very well.
"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 three performances."
Gatland will name his 30-man World Cup squad on Monday after finalising plans for a campaign that starts against South Africa in Wellington on September 11.
Stephen Jones (calf), Ryan Jones (calf) and Rhys Priestland (thigh) are all expected to be fully fit, with the biggest concern appearing to be prop Gethin Jenkins.
The Cardiff Blues forward is to see a specialist in the next few days regarding a calf muscle injury, although Gatland still retains hope he can play a part in the World Cup.
"Stephen Jones had a scan last night," added Gatland. "He felt the calf a little bit in training, but it is not a huge concern for us.
"Rhys Priestland not too far away, and Ryan Jones will train next week.
"There is a long way between now and playing South Africa in that first game.
"We believe we are in good shape physically, and we have won five of our last seven games. We have that confidence."
Flanker Martyn Williams captained Wales on his 99th appearance in what was his final Test match at the Millennium Stadium, and he must now wait to discover if he has done enough to ensure a 15-year international career has a World Cup swansong.
Williams said: "We spoke in the week as a group. For us today, it wasn't about individuals, it was about keeping the momentum going from last week.
"The boys who hadn't played didn't want to be seen to be letting the squad down.
"Argentina are a very tough side. They were third in the last World Cup, so it was nice to bow out with a win."
Argentina will provide England's opening World Cup opposition in Dunedin on September 10.
The Pumas were star performers at the last tournament in France four years ago, but there were inevitable signs of rustiness given that today's clash was their first Test match this year.
"That was our first Test for a long time, but I believe we made a step forward," said Argentina fly-half and captain Felipe Contepomi.
"In terms of the game plan and structure, we did well, but individual errors in terms of decision-making cost us.
"The more you play, the less errors you make, although that is not an excuse for today.
"I think we are moving in the right direction, we just need to solve these errors.
"We need to enter into the small details building towards the England game in the World Cup. Hopefully, we will arrive in a better way."
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