The Suffolk County district attorney’s office is appealing a gun case involving a Mattapan man after a judge overturned the jury’s guilty verdict on charges of possession of a loaded firearm without a license. Errol Reece, 21, of Mattapan, was charged with unlawful possession of a firearm and ammunition and unlawfully carrying a loaded firearm in 2010, said Jake Wark, spokesman for the Suffolk County district attorney.