Reason 1: Gaethje needs a knockout; how has he improved since his fight with Poirer and why has he been knocking Mfers out
Gaethje hasn't gained some serious power in his last three fights. In the last two gifs his opponents got knocked out because they were pressured against the cage with their hands down. I can tell you this won't happen to Tony because Tony doesn't get pressured. I'll explain further down below.
In this gif cowboy gets countered. Throughout the fight Gaethje was using his footwork to move in and out of range from different angles because Cowboy usually blitzes in with his chin up and throws two-three punch combinations. This is a tendency from Cowboy and he does this all the time.
So again, Gaethje hasn't gained some incredible power in his last three fights. This is a myth. He wasn't able to one punch KO Johnson, Alvarez, or Poirer because he couldn't land clean on them with good stamina. What has happened is he's become a better technical fighter. Instead of walking forward with his hands up he's been using his footwork and angles to cut off the octagon, push fighters against the fence, or counter, and land clean shots with good energy level. His striking has actually become incredibly similar, almost to Rose Namajunas, which isn't surprising considering they have the same coach Trevor Witman.
As you can see, Rose uses her angles and footwork to pressure Joanna against the cage, trapping her by throwing a good timed left hook preventing her from circling to her right, and finishing with a right in case she tried to go to her left.
Reason 2: Gaethje isn't a good finisher
As we see, when Gaethje uses his technical skills he can one punch knockout people, but as seen throughout his career, Gaethje isn't the best finisher. We never really have seen his top game, so in case he does land on Tony and knock him down its hard to know what will happen when he gots on top. Most likely since he doesn't have much experience with ground and pound in high level MMA fights, he won't do as good as Tony, who has shown he can recover even when he gets hit, and be a threat from the bottom.
In the Alvarez and Poirer fight he hurt both of them, but lacked the ability to finish them. Even in the Johnson fight he hurt Johnson and finished him, but Johnson was exhausted and just stood there, no footwork, movement, anything. Our best reference is the Alvarez and Poirer fight. Gaethje just hasn't shown the ability to finish high level opponents when he's had them hurt.
A counter argument could be is that in those fights Gaethje hadn't preserved his energy levels, so a good case could be that he can still knockout Ferguson in the first or second round. which leads to my conclusion
Gaethje can't knock Tony out
because doing so would require Gaethje to have better footwork and be better than cutting off the cage than Tony. which Tony is a master at and better. Look at the Pettis fight, and analyze how Tony switches stances constantly to prevent Pettis from circling out of his strikes. Every time he does, Pettis moves into his shot, Tony inflicting more damage. Tony is also good at Feinting and moving his head to give false reads to his opponent and make them strike inaccurate. He uses his feints to push them against the cage and has odd timing allowing him to strike when they are thinking about offense instead of defense This makes them get tired and pressured against the cage. Same in the RDA fight. Gaethje pressure is rather one dimensional, he relies on throwing leg kicks and clinching his opponent.
Here we see him clinch Johnson against the cage and beat him up. Here's the antithesis when he does it against Alvarez
we see him try to clinch Alvarez here and get knocked out; if he does this against Tony, he will eat Elbows and get cut rather soon.
Second part to my conclusion is Gaethje beating Tony would require Gaethje to have proved he is a better finisher than Tony is at recovering; I believe this not to be true.
so I believe all signs point to Gaethje getting elbowed, cut open, pressured against the fence, then panic wrestling and getting D'arced in 2:32 of the third round.