He said: "It is very frustrating and bitterly disappointing because for the third time this season a linesman has disallowed a perfectly legal goal.
"When they see the DVD they will see it's an embarrassing and horrendous decision.
"Gunnarsson only has his eyes on the ball and their player pulls him down for (what should have been a) penalty.
"Everyone has said the same."
The result leaves Cardiff outside the play-off zone following a run of just 12 points from their previous 13 games.
But, in a game of few chances, it was a precious point for Kenny Jackett's Millwall who edged further away from the relegation zone.
Cardiff striker Joe Mason saw his shot cleared off the line by Paul Robinson in a dire first period.
The home side came to life after the interval when Mason forced ex-Northern Ireland stopper Maik Taylor to tip his 47th minute header from Kevin McNaughton's cross over the crossbar.
Then Gunnarsson spurned a golden opportunity from close range when the midfielder failed to connect with Liam Lawrence's inswinging cross before Kenny Miller, who passed the 20 hour mark without a Cardiff goal in the eighth minute of this clash, was wide with a volley.
Millwall almost snatched three points with 15 minutes remaining, but James Henry dragged his volley wide of David Marshall's left post.
Moments later, the south Londoners had the ball in Marshall's net, but ex-Cardiff forward Andy Keogh saw his effort from Tony Craig's cross disallowed for offside.
Keogh was denied by a fingertip save by Marshall before Gunnarsson's header was controversially ruled out.
Millwall boss Kenny Jackett said: "It was a good point and we would have taken that before the game. It was a very spirited performance by ourselves.
"We played two up front and tried to counter-attack. We knew Cardiff would have a lot of the ball and we had to be brave.
"It nearly worked out for us at the end when we had chances. I'm pleased with a point."
Prior to kick-off, there was a minute's applause in honour of Millwall's record appearance holder Barry Kitchener who died yesterday from cancer.
Jackett added: "On behalf of the club, our thoughts and condolences are with the family. He was well thought of and well spoken about at the club."