Hi there. You will need to get your phone unlocked,Tesco phones are double locked.For your device to work on giffgaff,then it needs to be unlocked or locked onto the o2 network.There are a number of ways that this can be achieved.
1) Contact your current service provider and they can unlock it for a small fee.
2) You can take your device to any phone shop or market and also have it unlocked on o2,again for a small fee.
3) There is an unlocking service for most devices and there are also many websites than can unlock your device for free.
Please find below 3 links that can help to unlock your device thankyou.