Technically, you've been stealing any gas you've used.
Whenever you move into a new property, you have to enter into a contract for each of the services provided to that property. (If you use any of those services, prior to entering into such a contract, it's legally theft). That means that you needed to sign something! Somebody should shoot the letting agent because any decent agent should know about that. (i.e. it's not good enough for them just to tell the existing service providers that the tenancy has changed).
The previous tenants' contract, for the supply of gas, was automatically terminated when they left. So there's no current contract with anyone. (You were required to choose your own supplier when you moved in. There was no obligation to use the same company that the previous tenants had a contract with).
Since you're moving out at the end of June, you could just wait to see what happens. You might well find that you never receive any bill. However, since there's no contract, your gas supply could be cut off without any warning. Additionally, if it was clear that you'd never made any attempt to pay for the gas you've been using, it's possible that you could face a criminal prosecution for theft.
Chris