The marauding Maroons scored eight-tries-to-one to record the biggest Origin win of all time and secure their night series win in 10 years.
"We let ourselves down," a dejected Gallen said.
"Our training camp was unbelievable and I found myself today getting up thinking what do I do to encourage the boys because they have been so perfect.
"To go out there and play like that - it's not good enough. It's not good enough for the blue jumper."
Stunned coach Laurie Daley was still coming to terms with his side's capitulation at the post-match press conference.
"We were as confident as we could be coming in and sometimes that makes you nervous because you think everything's been too good," Daley said.
"And I think I'm just shocked that it ended up like that because I just didn't see it coming.
"But we've got to deal with it, we've got to accept it, the challenge has been set and we'll come back again."
Daley refused to speculate on the prospect of wholesale changes to his squad for next year's series and doesn't think the loss will have a lasting effect on the team.
"No I don't think so, if you know this group of players there's a lot of young boys in the team and it's a good lesson for them, it's a really good lesson," he said.
"They're going to be disappointed, they're going to be hungry and they've got to come back next year and have a crack."