issues. I would personally be VERY cautious about giving a gun to a co-worker or employee. But there are also several hundred people where I work. I don't know everyone personally- it's possible that someone has mental issues that I don't know about- or some sort of legal issue in their in their past. ALSO, I'd be weary about the PR consequences. With some of the shootings recently, a lot of people are sensitive. If there are a lot of employees at your office, you could offend someone- which wouldn't necessarily be a legal liability- but could still be a pain in the neck.
If your office is smaller- and you know (and trust) everyone- it might not be an inappropriate incentive. (I would STILL ask a lawyer.) If it's a bigger office, it might just be safer to give out a voucher, or the cash equivalent of the price of the gun, registration, a cheap gun-safe, etc. Or save yourself the trouble, and pick a different incentive.