Yes, Giffgaff think you should be free to do what you like with your phone, so every phone bought from giffgaff is 100% unlocked(SIM Free) and will stay that way So any UK or international SIM card will be accepted. This applies to iPhones, Android(Samsung, HTC, LG , Sony , Huawei) , BlackBerry, Windows Phone - the lot. They're all unlocked!
For more information on the process of buying from giffgaff check out this https://community.giffgaff.com/t5/Getting-started/Buying-a-giffgaff-phone/ta-p/10441209
Knowledge base guide.