Irish referee Rolland sent off Warburton for a dangerous tackle in the 9-8 semi-final loss to France at Eden Park.
Gatland admitted Warburton deserved to be penalised for illegally lifting France winger Vincent Clerc but Gatland felt the referee made a mistake by issuing the Welsh captain a red card.
"Having looked at it, we accept it that he lifted him and it warrants a yellow card, but he doesn't drive him into the ground," said Gatland.
"The thing that surprised me was the referee's instinct. I thought an experienced referee would bring in to the two touchies (assistant referees) and have a chat then make a judgement."
Warburton said he had no intention of driving the French speedster into the ground.
"I am obviously gutted with the red card. There was nothing malicious. There was no intent. I thought it was a normal tackle and the next thing you know I'm walking off into the stands."
Wales defensive coach Shaun Edwards added: "I think it's a travesty for the competition because the team that should be playing next Sunday isn't," he said. "(But) in times like this it's important to keep your dignity and not to say what you think."
"When you get two quality teams and one team goes down to 14 men then the other team should win the game comfortably," Gatland said.
"But we had some shots at goal and a conversion that gave us a chance and I can't be more proud of our guys in terms of what they achieved when they were down to 14 men inside the first 20 minutes.
"We've got to take it on the chin, it's no discredit to France because they didn't make the decision ...in credit to them they defended well. Our players ran themselves into the ground with 14 men, you can't ask for any more."