Handed a tough group which included champions and South Africa and the dangerous Samoa, the Welsh needed to win their final match and did so with ease.
"That was the target coming here. We knew we were in a really, really tough group and the first goal was to try and win a few games in this group and make the quarter-finals," Gatland said.
"We've achieved that so it's one game at a time now. We worked really hard in the off season, and we said we're too big, too fit, too powerful and hopefully too smart not to progress, so we wanted to be ruthless.
"I'm really proud; tough conditions, but I thought the boys went out and gave a really balanced performance."
His side's only defeat of the tournament so far was a 17-16 reverse against South Africa.