Most sims are activated in up to 4 hours, but it can take up to 24 hours.
Please try turning your phone off and back on every 30 mins as this sometimes helps.
If not once the 24 hours are up and it has still not activated you will need to contact an agent.
As this is a account related issue you will need to speak to an agent, you can do so by clicking this link of https://support2.giffgaff.com/app/ask/
giffgaff agents will do there best to get back to you within 24 hours but at busy periods it could be slightly longer.
If your request has not had a response within the 24 hours you can click this link below to ask an educator to chase it up for you. https://giffgaff.com/support/questions