Found a gun, what to do?

Actually, you didn't truly find a gun. You know who the firearm belongs to. Under the abandonment laws (in most states), you have a right to sell the tenant's non-personal property (furniture, etc) to recover your losses (unpaid rent, etc), but may not keep property or proceeds exceeding that. Any excess property or proceeds (including personal documents, restricted materials, etc) must be mailed or shipped to the tenant at the tenant's last known address, or to any next of kin known or listed on the rental contract. I fully suspect a firearm falls under the latter category.

 

If you do property management, you probably have an attorney. Ask him/her. Failing that, report it to your local law enforcement agency.

 

If you have a connection at a pawn shop, they can run the number to check if it's stolen. I don't know if they would charge for the service.

 

4473 is required on any FFL transfer.
The police would be acting as an FFL in this situation and they will process a 4473 (and background check) before handing the gun over to the OP.

 

Or disassemble and destroy it. The safest path is probably notification of local police and follow that path to attain legal possession.