The newscaster gave her take on "CBS Mornings" Wednesday after a segment about JT's arrest ... saying he's a great guy who isn't usually a reckless person -- and, calling his alleged slip-up a "stupid mistake."

Gayle adds there's no excuse for driving drunk in general ... and says she's sure Justin knows he messed up.

BTW ... GK isn't using words like "allegedly" when sharing her thoughts here -- but when one of her co-anchors pointed out that this is just what cops are claiming, she acknowledged that what they say they saw as probable cause to arrest him didn't sound very good.

As we told you ... Timberlake was arrested for DWI early Tuesday morning in Sag Harbor, New York after cops claimed he blew through a stop sign and swerved between lanes.

Cops say Justin told them he had only one drink at The American Hotel in the city, but they say Justin exhibited glassy/bloodshot eyes, a strong odor of alcohol and slurred speech. They also claim he failed field sobriety tests ... and refused to blow in a breathalyzer.

Justin spent the night in jail before being released without bail after his arraignment. His lawyer Edward Burke Jr. says they plan on fighting the DWI charge tooth and nail.