If you’re willing to sign a contract for with your gas company, you’ll pay less for each unit of gas you use than if you choose to go month to month where your rate can change at any time and you will be paying the most for each therm. A fixed rate in this case does not mean the same bill each month though. It still would vary based on how much gas you’re using. Think of it like the prices on a gas pump per gallon. With a contract the per gallon prices would stay the same every time you drive up for more. However if you break your contract by moving early or something you will have to pay a fee.

Editing to add: the gamble with choosing the 6 month plan is that the rates can go up when you are ending your 6 months and you may end up having to pay a higher rate next time