Hello have you recieved any emails saying from giffgaff and asking you for details or to login in if so you have been scammed never login other than app or go to your normal login method ,
giffgaff will never give you a link for details ,
you must change your password ,
Hope you got the PM I sent you. My advice due to the holidays is to perform a SIM swap and take control back of your account.
This can be difficult if you don't have a spare SIM. You already have the link for 'giffgaff in shops' but you might be able to get one hand delivered or information on a local shop.
Pop in your local area and with fingers crossed you might have a giffgaffer local to you.