The winner of the Australia and England game in Cardiff on October 26 is almost certain to top Group A and will enjoy an easier run to the final than the loser, who are almost certain to have to face World Champions New Zealand in the semi-final.
"It's a massive game first up in Wales, hopefully they'll be on our side so we get a bit of support," said Gallen.
"You'd like to think that if you win the first game you're a fair way on the way to the final.
"It's a huge game and England have a very good so and we're going to have to be ready.
"They've got some big forwards and obviously the experience of the Burgess boys playing in the NRL.
"If we want to win, we've got to get the job done."
If Australia do meet New Zealand during the tournament, they will likely come up against Kiwi superstar Sonny Bill Williams who was a controversial replacement for Melbourne Storm forward Tohu Harris. Williams declared himself available for his nation after the 24-man squad was confirmed.
"It's a huge deal," said Gallen. "You want to play against the best and you want to see the best there.
"I can't blame New Zealand for picking Sonny Bill, I feel very, very sorry for Tohu Harris.
"The world wants to see Sonny Bill there, it just would have been nice if he was there from the start."