"So, they haven't paid the gas bill?"
Usually, yes. But it could be any form of financial or trade debt to British Gas, including (for example) repayment for services Alphadex contracted to deliver that were paid but not delivered, or delivered but of unsatisfactory quality, leading to a claim. Either way, once a company starts getting winding up petitions its days are usually numbered, because in addition to a refusal or financial inability to pay debts, there's often a systemic problem about how they've got to the stage of not paying bills.
The court doesn't have to grant the petition, and as an unsecured creditor you'd usually get nothing from winding up a non-viable business, so typically winding up petitions are used as a way of escalating a disputed debt where you believe that the company can pay, but is choosing not to.