UFC's Justin Gaethje: I'm Done After 30 Fights

Undefeated UFC star Justin Gaethje is saying sayonara to MMA after another dozen fights, tops ... telling TMZ Sports the sport's "too brutal" to gun for 50-0 like Floyd Mayweather.

"I'll get to 30-0 and I'm outta here," Justin told us ... before saying there's NO WAY a UFC fighter could hit 50 like Floyd.

Gaethje would know -- dude's got the second longest undefeated streak in the biz at 18-0 ... and is looking to make it 19 when he fights rival "TUF 26" coach Eddie Alvarez.

Justin says he'll wreck Eddie to stamp his ticket to a Conor McGregor fight ... and told us Conor's legitimately "terrified" of that matchup.

With Nate Diaz demanding CRAZY cash ... Gaethje might just get it.