"It was the most terrifying night of that man's life."

Yeah, he ain't lyin'.

Gaethje joined the guys on the "TMZ Sports" TV show (airs weeknights on FS1) -- and said it was actually a "scary moment" for Vick ... 'cause getting KO'd is no joke.

"I've been there man. It's scary actually. His brain ... it's like a reset button on your forehead. It's like someone presses it every 30 seconds."

Gaethje was the #7 ranked lightweight fighter in the UFC going into the fight. Vick was ranked #10. The fight ended 90 seconds into the first round when Justin threw a crushing overhand right.

Gaethje says he actually felt bad for Vick -- a little bit -- but in the moment he was focusing on his accomplishment and "it feels f**king amazing."

Despite all the trash talk before the fight, Justin says there's no hard feelings anymore -- because it's way easier to squash beef after you almost knock your opponent's head off.

"It's a painful way to go out."