If the goodybag has activated agents are not obliged to give refunds as it's your error.
However, if you have used very little then an agent may give you a refund at their own discretion.
You can raise a case with the agents via the support link in your My giffgaff page messages section. https://giffgaff.com/support
Agents can take up to 24 hours to reply, but are usually much quicker.
Agent replies can be found in your my giffgaff page messages section.