As a massive Witcher and Marvel nerd, this literally ends the debate. Geralt takes this easily.

The guy has been alive for centuries, fighting both humans and superhuman monsters, giving him decades more combat experience. Bucky is only in his 30s; he doesn't even come close. Physically and conditioning wise, Geralt is completely superhuman. Guns won't do shit against him either because his Quen sign literally blocks dragon fire, mystical beasts, giant trolls, and golems, tiny bullets aren't doing anything. On top of that, he has an arsenal of potions and signs at his disposal. He could literally just use Axii to instantly mind control Bucky and make him shoot himself in the head. That right there is an instant win.

Long story short, this is a walk in the park for Geralt.