Simple solution. Speak to a local attorney and ask that they turn the gun in for you. They should be willing to do this for a very small fee and you will be covered by the attorney-client privilege. Otherwise you could attempt to turn it in anonymously but that may be more difficult. Further, if you've touched the firearm then you may have left your own latent prints on the weapon. If not, then don't touch it.
If you decide to turn in the weapon yourself, make sure your attorney is present during any statement you give. If you cannot afford one, I recommend you first visit your local public defender's office and explain what you need to accomplish. They should give you some guidance to help.
Obviously I don't know the full details of this matter but it is likely that turning this weapon in would make a witness out of the person against the person involved in the crime who is trying to dispose of the gun. If the "someone" who was given the gun attempted to dispose of it, they would likely be committing a crime (or at least become an accessory to the one already committed).
Consult a lawyer in your area to handle this issue. It is the best way to prevent any unnecessary harm.
Please reply if I can help further.
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Criminal Justice Lawyer
Over 11 years of practice in litigation including 10 years as a state prosecutor