Gatland's side were by far the more dangerous of the two but couldn't break through the solid French line to edge ahead in the game.
"I just feel like the destiny of the result was taken out of our hands with the red card," Gatland said after the game.
Flanker Warburton was adjudged to have dangerously tackled French winger Vincent Clerc but replays indicated that the decision was harsh.
"I can understand he's lifted him, that's a yellow card but he hasn't driven his head into the ground," Gatland added.
"Does that mean from now on that every time there's a tackle made and you lift someone that it's got to be a red card?
"I just feel gutted, why spoil a World Cup semi-final by giving a red card?"
The French will take on the winner of the tomorrow's second semi-final between Australia and New Zealand in next Friday's final.