Is there a safe, ethical, way to get rid of old gas?
If you do not wish to risk using it, even though it is probably only 6 months old, I would take it to where ever your community recycles hazardous wastes.  In my community, the recycle center has the following information on it's website:


  Fuels and fuel oils are petroleum products that are used in many types of engines, lamps, heaters, furnaces, stoves, and as solvents. Fuel oils come from crude petroleum and are refined to meet specifications for each use. Fuel oils are mixtures of aliphatic (open chain and cyclic compounds that are similar to open chain compounds) and aromatic (benzene and compounds similar to benzene) petroleum hydrocarbons. In addition, they may contain small amounts of nitrogen, sulfur, and other elements as additives. Fuel oils are distinguished from each other primarily by their boiling point ranges, chemical additives, and uses. Fuel oils include: Kerosene, coal oil, diesel fuel, home heating oil, and Gasoline. All fuels and fuel oils should be disposed of as a hazardous waste.


In this community there is no cost for this service.