The 32-year-old had won the Australian national championship road race, the Tour Down Under and the Milan-San Remo classic early in the 2012 cycling season.
Now he's added a victory in a tough late-season event on the UCI World Tour that showed he is on form heading into the world championships this month in the Netherlands.
"I've put a lot of emphasis on being in good shape for the end of the season," Gerrans said after taking the hilly, 201.6-kilometre race in four hours 53 minutes four seconds, with Van Avermaet just behind.
"These races in Canada are part of that goal. I'm thrilled to win here. The world championship is only three weeks away and my conditioning is going up."
More than 20 teams, including many of the world's top road racers, were already packed for the move to Montreal for a second World Tour race up and down Mount Royal on Sunday.
Rui Costa, the Portuguese leader of the Movistar team, led in a hard-charging pack to take third place. Costa is the defending champion in Montreal. Luca Paolini of Italy was fourth.
"The Quebec circuit is the more difficult," added Gerrans. "It's stop-start all day.
"There's no place really to recover. It just worked out well to attack in the final (lap)."
It was a tough day for Giro d'Italia winner Ryder Hesjedal, as the Victoria, B.C., in his first competition since the London Olympics, ran out of steam on the final lap and finished 94th
"As a whole, I just didn't have the real power for the final," the leader of the Garmin-Sharp team said. "It was a hard day — close to five hours and this course just doesn't let up.
"I haven't raced since the Olympics and it takes a while to get to your best level. But I'm happy with my race. Hopefully on Sunday I'll be better."
In near-perfect 23 C weather, Quebec City was at its stately best as the riders strained through 16 laps of a circuit that took them through the Plains of Abraham, along the St. Lawrence River, into the historic old town and back to a start-finish line on the tree-lined Grande Allee.
The first 15 laps, which included a long breakaway by an eight-man group that included Victoria's Marsh Cooper of the Canadian National Team and Hugo Houle of Ste. Perpetue, Que., served only as prologue for a wild final 12.6-kilometre lap.
The pack had already begun to splinter when Gerrans and Van Avermaet shot into the front to take a short lead.
In hot pursuit was one of the race favourites, including Slovak Peter Sagan of the Liquigas team, who looked like he would overtake the two leaders on his own until the last 100 metres before the finish line.
An exhausted Sagan, who finished 26th, saw a pack of 16 led by Costa fly past him and finish only four seconds short of the leaders.
In that group was Francois Parisien of Montreal, who was top Canadian in 10th place and first among three members of Steve Bauer's Spidertech team in the top 25, along with Ryan Roth of Cambridge, Ont., in 23rd and Guillaume Boivin of Montreal in 25th.
"It was pretty nerve wracking," said Gerrans, who led the Australian Orica Greenedge squad. "Both Greg and I worked hard to get that gap and we worked well together to maintain it to the finish.
"We couldn't really slow down much before we sprinted to the line because the peloton was close behind. One of the strongest guys, Peter Sagan, was between us and the peloton, so we had our work cut out."
Van Avermaet, the 2011 Tour de Wollonie champion, said Gerrans was "a little fresher" at the finish.
"He did a good sprint and he deserves the victory. I'm happy with second," the 27-year-old leader of the BMC Racing squad said.
It is the third running of the only two UCI World Tour races held in North America and no rider has yet to sweep both events, although BMC's Philippe Gilbert won in Quebec City and came third in Montreal last year. Gilbert is entered in the Vuelta this year and won the 19th stage of the Spanish tour on Friday.
It was a good day for Canadian riders. Bruno Langlois of Quebec City, who was 30th, was named best climber of the race. David Veilleux of nearby Cap Rouge, Que., made a breakaway attempt on the final lap but finished 28th.
Thomas Voeckler, the 2010 winner, came in seventh, just ahead of Hesjedal's German teammate Fabian Wegmann.
Another favourite, Sky Procycling leader Edvald Boasson Hagen, dropped out just after the halfway point. The Norwegian just turned his bicycle into the headquarters hotel, the Chateau Frontenac, which was on the race route.
Hesjedal is looking forward to a better race in Montreal, where the course is steeper but each lap has only one long climb.
"I hope this brings me up for Sunday and for the worlds and the last Italian classics," he said. "It's a nice place to race and try to build form."