Police spokesperson, Priscilla Naidu, says the gun will be sent for ballistic testing.
Mark Minnie’s body was found near a rose plantation with a bullet wound to the head on Monday night.
Naidu says a suicide note has also been found on the farm at Theescombe outside the city.
Minnie’s cell phone and car are still missing.
The book that was released at the beginning of the month implicates a prominent businessman and two former National Party ministers, including former Defence Minister, Magnus Malan, in an alleged paedophilia ring that operated during the apartheid era.
The book alleges the trio molested and raped minors at Bird Island off the coast of Port Elizabeth.