McGregor is reportedly preparing to fight Dustin Poirier early next year, after performing another U-turn after a third retirement.
But Gaethje says if McGregor wants the belt, it’s him he’ll have to defeat.
“With him retiring, I'm No. 1," he said. "Poirier and McGregor are going to fight. They haven't made it official, so if McGregor wants to bounce out and get the belt, then I'm right here.
“If they want to give it to someone else, then so be it. But I'll be ready. I'm ready to fight again. This is my job. This is my life. I love it so much.”
McGregor had tweeted a scathing assessment of Gaethje’s performance against Nurmagomedov, suggesting his “overall fear of the central positions being truly embarrassing”, though the message was later deleted.
Gaethje also suffered a TKO loss to Poirier in 2018, before embarking on a four-fight win streak which was halted by this latest defeat to the now-retired Khabib.