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, "Gangster's fight, they don't run their mouths."
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 .