When you port out from giffgaff your SIM card will be deactivated and active allowances e.g goodybag and/or airtime credit will be removed from the account. However, if you wanted once your number has been transferred over to the new provider you can activate a new SIM(top up required) on the existing account and you will be allocated a new random number.
You can obtain the PAC code online here http://www.giffgaff.com/profile/payment-details
to avoid any future charges e.g. recurring goodybag.