Is your phone unlocked or a o2 phone because giffgaff runs on the o2 network.
If your phones a iphone try this.
If the iphone is unlocked / locked to O2, and is showing anything similar to 'No service' or 'No SIM', try this:
Insert giffgaff SIM - do not turn on (if SIM is already in, turn off)
Connect phone to computer with USB cable
iPhone will switch on. Wait until itunes registers it and checks for updates
Install the carrier profile update if offered
Disconnect and restart phone