Nate Diaz's tough guy image is just a front, 'cause he's all bark, no bite ... according to UFC star Justin Gaethje ... who says, "Gangsters fight, gangsters don't run their mouth."
TMZ Sports talked to Gaethje -- who's fighting Edson Barboza (that fight is going to be ridiculous) Saturday on UFC on ESPN 2 -- when JG opened fire on the Stockton fighter.
"The only thing that's been bothering me, Nate Diaz keeps talking so much shit but the dude won't fight," Gaethje tells us ... "It just amazes me how people think he's still a gangster."
"You can't talk and not show up. That's just funny to me."
Nate hasn't fought since losing to Conor McGregor in the rematch of their blockbuster fight ... and that was 951 days ago.
Dana White says he's offered Diaz a bunch of fights ... only to be turned down. Nate did agree to fight Dustin Poirier last year at UFC 230 ... but the fight was scrapped after Poirier got hurt.
Gaethje thinks Diaz has been running his mouth (it's unclear exactly what has Justin pissed off) ... so, we asked the logical question ... do you want to fight Nate?
If JG's right, don't hold your breath ... 'cause Nate ain't interested in anything more than running his mouth.
"He ain't fighting nobody. That dude ain't fighting nobody. He's all talk. He wants to keep this perception that he's a tough guy through his words. He don't wanna fight no more."