A couple different videos from the Republican National Convention in Milwaukee show Gaetz heckling McCarthy as the former congressman gives a TV interview ... interrupting him while he was speaking and mockingly wondering if he was set to speak Tuesday night.

Gaetz -- who led last year's charge to oust McCarthy as House Speaker -- can be overheard taunting KM over his lack of a speaking slot at the RNC ... at which point Gaetz drops the hammer, saying ... "If you took that stage, you would get booed off of it."

It might've felt like a sweet W for Matt -- but just a second later, as he's turning around, he's confronted by an older gentleman who gets right in his face ... and tells him a thing or two.

Check it out ... the dude tells Gaetz -- whom he addresses by last name, BTW -- to stop being an "a**hole" ... and you can tell Matt's thrown off by the whole interaction. MG says he doesn't even know this fella, but the senior citizen holds his ground ... staring him down.

Eventually, Matt shimmies his way around and gets out of the tense situation -- but this is literally almost right out of a movie ... a strong-willed do-gooder telling a bully to buzz off, and as a result -- some think the nameless guy here got the better of Matt in the exchange.