Are anesthetic's only in gas form and do they have a chemical composition if used as a gas and not a pill?

No: Anesthetics like those used to sedate you for surgery and procedures can come in gas OR liquid form and are often injected into an IV before a surgery or procedure. Dentists tend to use gasses rather than the injectable liquid forms due to the process of inserting an IV. I'm not sure what you mean by do they have a chemical composition... any medication in any form has a chemical composition.

Many forms: Anesthetic gas is a long term agent. There are intravenous forms , liquid forms used in spinal and epidurals as well as nerve blocks ( dentist office for instance) and pill forms as in narcotics, valium etc.