And Roberts went over again after the break with Lloyd Burns, Leigh Halfpenny, Lloyd Williams and Jonathan Davies all adding scores of their own against a weakened Fijian side.
And after suffering defeat against defending champions South Africa in their first game of the tournament, Gatland is adamant his team have now turned the corner.
"We knew coming here that this was the toughest pool and our first aim was just to get out of it. We've achieved that goal," he said.
"And since losing to South Africa in such a close encounter, we think we've developed a bit of momentum and a bit of confidence going into the quarter-finals.
"We feel that coming out of this pool it's a little bit like South Africa at the last World Cup, if you come out of a tough pool, it does potentially set you up for the quarter-final and maybe a little bit further.
"We're under no illusions that Fiji made ten changes and so we have to be careful not to get carried away with the result and the performance and keep things in perspective.
"One of the most pleasing things was that it would have been easy to slacken off and give up a few tries but we kept going until the 80th minute."
North was the real star of the show for Wales, running in a try of his own while playing a crucial role in at least three more as he seemingly broke the Fijian line at will.
And at only 19 years old North is a prime example of the new generation of Welsh rugby player that Gatland believes are making big names for themselves down under.
"We arrived in New Zealand with many people not knowing much about some of our youngsters," he added.
"I'd like to think one or two of them have made something of an impression since they've arrived here.
"There's some fantastic youngsters who are really starting to make names for themselves."