I'm curious about how Gunslinger's second attack interacts with Reckless Diversion.

Reckless Diversion:

" When an enemy unit performs an attack, it must attack a trooper unit that has a faceup order token, if able."

So, my opponent plays Reckless Diversion, and I play Sorry About the Mess. There's multiple targets available for my Han to shoot that don't have a face-up order token. Do I forfeit my second attack from Gunslinger, or can I shoot something without a face-up order token because Han is no longer able to be attacked?

If for any reason you can't attack a unit with a face up order token, then you can attack another unit. In this case, you can't attack Han because Gunslinger let's you make a second attack but must be a target different from the first one, so here the "if able" part of Reckless Diversion kicks in. As you are not able to attack Han, you can attack another unit.

1 hour ago, NoShieldsAllGuts said:

Do I forfeit my second attack from Gunslinger

This would only happen if the card instructed you to. If it said something like "it may only attack a trooper unit that has a face up order token," then you would not have any legal targets. This just says "You have to make this choice, if you're able." You're not able, so you're free to choose something else.