Montgomery County District Attorney Risa Ferman has ruled that an Upper Frederick Township man acted in self defense when he shot two men outside his home last December.   42-year-old Angel Gonzalez had the gun.   Investigators say he fatally shot 19-year-old Zachary Levin and wounded his stepfather 34-year-old Joshua Levin after the two men attacked him with baseball bats.   Gonzalez was living with the estranged wife of the older Levin at the time of the shooting.