Brand new iPhones sold by giffgaff come with the standard 12-month Apple warranty. Please refer to Apple guidelines for their exact warranty information.
Any refurbished phones come with a 12-month warranty(giffgaff only) and pre-owned phones come with a 6-month warranty(giffgaff only). Please note standard warranty doesn't cover accidental damage or lost/stolen devices.
More details on phone warranties can be found in this https://community.giffgaff.com/t5/Using-giffgaff/giffgaff-Handset-Warranties/ta-p/19936387
knowledge base link.