What is the ruling on selling gigabytes to a customer as if they were complete, despite my knowledge of their deficiency from the service provider, or should I reduce them for the customer and bear the loss myself for fear of falling into sin?

You must fulfill the data volume agreed upon with your clients (a gigabyte for 2 pounds), regardless of your dealings with the service provider, because your contract with the clients is separate from your contract with the company. Allah Almighty says: "O you who have believed, fulfill [all] contracts," and the Prophet, peace and blessings be upon him, said: "Muslims are bound by their conditions." If you are harmed, you can amend your agreement with the clients and clarify that the data reaching you is less, and deliver it to them as it reaches you. If they agree, there is no blame, or you can adjust the rental value to complete the agreed-upon volume for them without harm. You have the right to demand your right from the company that tampered with it.