Well, we know a good hit from the Dragon Slayer will likely crush any defence Kirito puts up, might smash his sword at the same time, and will leave him with serious damage. After that Guts will keep attacking, and with Kirito wounded and in the ground I don't see him recovering quickly enough, and even if he does he can't do much at that point with his weapon gone. So if Guts lands the first hit with his range advantage Kirito is done, due to the size of the weapon and Guts' AP advantage.
So if Kirito wants to win he has to dodge that initial attack and immediately close in to attack with his sword. Guts will block that with his metal arm, and at this point Kirito has a good shot at taking off the metal arm or taking the Dragon Slayer via his superior lifting strength. If he takes the Dragon Slayer Guts will respond by punching him, and with Guts' superior AP that will hurt a lot, allowing Guts to follow up with more punches from his regular arm and his metal one, leaving Kirito badly beaten very quickly, at which point Guts will kill him either with the arm or by taking back the sword and using that. So Kirito's best move is taking the metal arm once he's in close range and stabbing Guts with the shorter sword. Even then Guts can still attack with the Dragon Slayer or bring the Dragon Slayer in close to block, but with the arm pulled off Kirito actually could hold back the Dragon Slayer by gripping its hilt and take the Dragon Slayer at this point. Even then Guts will retaliate by punching him which will hurt, but Kirito will retaliate with his sword, and if that happens Kirito would indeed take the win.
This might sound like a viable strategy, but it's actually just listing Kirito's best outcome. First off Kirito has to make two correct choices in a row, first to dodge the sword rather than block and second to take the metal arm first. If he makes the wrong choice either time he's done. I also haven't talked about what Guts would actually be doing during the attempts to do this. First, speed being equalised means that even if Kirito dodges the first hit from the Dragon Slayer, Guts can retreat as Kirito advances, allowing Guts to keep distance between them to capitalise on the primary function of a weapon like the Dragon Slayer: its reach.
This means Kirito would have to dodge multiple attacks to reach Guts. If Guts delivers vertical attacks Kirito can sidestep, but sweeping horizontal attacks require he either jump, allowing Guts to punch him when he gets close, bringing us back to the AP beat down from earlier, or slide under it allowing Guts to either kick him or bring the Dragon Slayer down while Kirito is on the ground and has his movement impeded. Also keep in mind Guts will be accustomed to using the Dragon Slayer exactly this way. The main purpose of a long weapon is keeping your enemy away from you, and Guts has been using a very long sword for most if not all of his adult life.
TL;DR: So overall, even if Kirito, who has no way of knowing he can't block Guts' attacks or how hard Guts can hit, makes the right choices, I say Guts still has the best chance via superior reach, superior AP and superior skill/experience. Kirito's best chance is his lifting strength advantage, and even if he uses it to disarm Guts, Guts can still bring the pain without weapons via his AP advantage.
