Okay here goes. I have no idea who this Gerald the Witcher is, but I have watched the Predator movies.
I pick Predator. He can turn himself invisible, has a weird weapon laser blaster thingy on his shoulder. He can change his vision to do something to be able to see what's moving in front of him in any light. The Predator also has the ability to heal himself if hurt. And worst case scenario if trapped can unleash an explosion equal to that of an atomic bomb.
The other guy won't even know what side Predator is coming from.
If you want to learn about the competitors, we've written short, easy to read bios giving brief backstories and a list of their powers/abilities. Geralt of Rivia has a nice addendum from NorCal with much more information than the short intro provides.
Now here's why you're wrong:
Geralt of Rivia has faced monsters very similar to the Predator before. Sure, they didn't have plasma blasters mounted to their shoulders, but they could turn invisible, excel at hand to hand combat, and stop at nothing to defeat their prey.
And yet, Geralt has slain them all.
Slaying crazy strong monsters is what Geralt does, and Predator probably wouldn't even crack the top five list of crazy strong monsters that Geralt has killed. He has enhanced senses that allow him to track monsters that would evade detection by normal humans, he has enhanced reflexes that allow him to avoid attacks normal humans could not, and he has enhanced strength that allows him to penetrate through armor that normal humans would not be able to penetrate. He has magical spells that he uses to give himself a sturdy overshield, trap the enemy in place, confuse the enemy, throw the enemy off their feet, or light the enemy on fire. And he has tons of potions that give him buffs or heal him, and multiple bombs that he can use to freeze, blind, or flat out blow up his opponent.
Each of these competitors is equipped with an impressive collection of gadgets, but when it comes down to it this is about a hand to hand fight between two gifted combatants, and Geralt's Witcher mutations along with his experience and proficiency at swordplay are what give him the edge over Predator.
Vote Geralt.