From Wikipedia:

"A CBS camera caught a disgusted Gumbel blurting out, "What a fucking idiot," just after he had finished a hostile interview with Robert Knight of the Family Research Council (FRC). The incident occurred at about 7:15 a.m. ET on Thursday, June 29, 2000, following Knight's appearance to defend the Boy Scout policy of excluding gays from being leaders. The Media Research Center reported that he uttered those words; Gumbel openly admitted to saying so when guest-hosting a June 2007 episode of Live with Regis and Kelly."

The Media Research Center's page on the incident, which includes a now-unplayable RealPlayer media file of Gumbel's alleged use of the word "fuck," is below:

http://archive2.mrc.org/biasalerts/cyberalert-06302000-fing-idiot#1

Unfortunately, all I could find was this screenshot from the 2000 broadcast from the aforementioned page:

http://cdn.mrc.org/archive/stillshots/2000/gumbelslip.jpg

However, I do remember downloading it and being able to view it years ago. I downloaded the clip again recently, but am unable to get it to play on several media players I have tried. I've searched for it on all the usual video-sharing websites, but have come up completely empty of finding anything.