Yes - just make sure you log in to your account with the active SIM first then you won't be required to buy a goodybag
As you are already on giffgaff just use the current SIM card if it fits. If it doesn't, and its not a 3in1 SIM the phone will have a new 3-in-1 trio SIM in the box when it arrives. You can use this SIM to do a SIM Swap if you are an existing giffgaff customer, which is really easy to do. (Just pop out the right size SIM)
All your account details will transfer over to the new SIM in full within 30 minutes, including goodybag, number and credit.
● Remember to back up your SIM contacts beforehand
Full giffgaff guide to SIM swap here -