How do I dispose of gasoline?
Your local fire department should take them off your hands and dispose of them properly (in an environmentally safe manner) and safely.

I say the above, as I recently wanted to dispose of some old engine coolant in a responsible manner and didn't know where I could take it for disposal, so I went to our local fire station to ask them, they pointed me in the right direction and additionally informed me for future reference they take and dispose of petrol (gas).