Kathie, if you had a bad experience with the gas years ago, apparently your concerned about it now. Because of this your sub conscious mind will drag this up into your conscious mind and again it will be difficult going under. I would suggest seeing a hypnotherapist and have a couple of sessions one for the stress of going under along with relaxation techniques, and to Learn self hypnosis where you can visualize the procedure to overcome your fear.
Just so you know, in the medical field you can have surgery without anesthesia, but would need a number of sessions to be comfortable with it. Just to give you some facts, it is used in the OR by a qualified hypnotherapist and even in the dental office for root canal, etc. In fact, hypnosis has been used to control bleeding, and in emergent situations at disaster scenes. Talk to a qualified hypnotherapist, go online, you can find a directory of who might be in your area. I have had much success with clients who use hypnosis to overcome this fear.