Brisbane finished 2-1 winners thanks to a Besart Berisha brace, and the Spanish coach said the Reds began to rush their build-up play after the second goal went in.
United were able to disrupt the Roar's possession game for much of the first term, and had 74 per cent of the ball in the 22nd minute when Berisha's first goal went in.
The real turning point came with the Albanian's second strike early in the second half, according to Gombau, when Adelaide began to push too hard for a response.
"I think that in the first half we played better than in the second," Gombau said.
"And I think that what changed, maybe, the way that we played, and made it worse, was the second goal.
"Because when you start to lose 2-0, at that moment you want to arrive to the last (attacking) third faster, because you feel that you don't have time.
"And at that moment you start to play more long balls and you have less possession of the ball and this is a mistake, because in the end we are training to build from the back."
Speaking about the ball which appeared to cross the line from a Thomas Broich long-range shot late in the first half, but was not awarded as a goal, the Adelaide manager pointed out that such decisions tend to even themselves out.
He did, however, endorse the use of goal-line technology in football.
"This is something very difficult," he said.
"Also, I don't know, but the first goal for me, the second pass that made it can be offside, and the referee (gave) one for the other for me.
"If in the normal life we use technology, why not in football?
"This is my opinion, because if it's a goal, it's a goal. Finish
"Because the referee needs to decide in one second, and it's very difficult.
"These things, we need to help, and make the game be more clean."
