@juvedemis giffgaff SIMs are sent out first class and and should take no more than 3 working days to arrive. Check your address details are correct under your details page here http://www.giffgaff.com/profile/details if your address details are correct and it's been more than a week and the SIM hasn't arrived raise a support ticket with a giffgaff agent to investigate.
To contact an agent follow the link below
https://support2.giffgaff.com/app/ask/Getting-Started/My-SIM-hasnt-arrived/form/ , please note agents may take up to 24 hrs to reply to any queries.
giffgaff agent replies can be found in the “Messages From Agents” box on your “My Giffgaff” page or you can access it directly by following this link https://giffgaff.com/support/questions