Tom Adeyemi gave the visitors an early lead before Callum Harriott was shown a red card for a high tackle on Craig Noone. Charlton felt the visitors should have been reduced for 10 men, but Sean Morrison was shown just a yellow after he pulled down George Tucudean when through on goal. However Charlton kept fighting and Gudmundsson levelled two minutes before full-time.
It was a slow start from Charlton as Cardiff snapped into the early tackles and denied them any time on the ball. The pressure soon told and home goalkeeper Neil Etheridge had to act swiftly to gather a Peter Whittingham corner just as Kenwyne Jones was about to pounce.
The opening goal arrived on 11 minutes. Aron Gunnarsson launched a long throw into the box and Sean Morrison rose well and flicked on at the near post before Adeyemi placed his header expertly beyond Etheridge.
Just two minutes later and the same tactic almost worked again. Gunnarsson's throw into the danger zone was half-cleared and Adeyemi fizzed a right-footed half-volley narrowly over from 20-yards.
The Addicks pressed forward but rarely troubled David Marshall in the Cardiff goal. Harriott, Charlton's pacey winger, had some joy down the right against Declan John but when he wriggled free and powered towards the box his cross was well intercepted by Ben Turner.
It was Harriott's final contribution. On 32 minutes his ill-timed tackle on Noone was late and high. Noone crumpled to the floor in agony and referee Mick Russell quickly flashed his red card.
Charlton responded well and created their best chance of the game on 38 minutes. Yoni Buyens played a clever ball into the box and Charlton's Romanian striker George Tucudean fired in a powerful near post volley that Marshall was forced to beat away for a corner.
The hosts created the first opening of the second half on 52 minutes. Buyens picked-out Gudmundsson in the box who turned well but shot straight at Marshall.
Charlton, playing with 10-men, continued to press. The impressive Gudmundsson whipped in a corner and Tucudean's near post flick rattled the outside of Marshall's post.
Then came the controversy. On 63 minutes Morrison, the Cardiff defender, hauled down Tucudean who was through on goal. This crowd chanted for a red but the referee issued a relieved looking Morrison a yellow. To add to the agony, Gudmundsson cracked the post from the resultant free kick.
Marshall then denied Gudmundsson with a fine save on 70 minutes before Morrison cleared off the line after Tucudean's close-range flick from Jordan Cousin's shot had beaten Marshall.
However Charlton gave it one last push and with two minutes left were rewarded. Gudmundsson picked up the ball 30-yards out and for once Cardiff's defence failed to get tight. The Icelander took aim and lashed home a superb left-footed effort into the top corner.
And it could have been even worse for Cardiff had Igor Vetokele not skied an effort way over the bar when he was clean through on goal with the very last kick of the game.