Hi @djmixter sorry but giffgaff can't tell you who's ringing you if the number is withheld. You could block such numbers using a number blocking app but this will block a ligitimate call from a withheld number too. However try blocking them for a short while to see if they get bored.
@djmixter What phone do you have? It may have an in built option to reject unknown numbers? IF not, then as somebody else suggests, download an app that will block all unknown numbers.
If they do not get bored and keep on ringing, then you can always request a new number from giffgaff here https://giffgaff.com/profile/details#msisdn but if you do this, you cannot retrieve the old number.
Also be aware, if you use an iPhone, people can contact/message/phone you via the Apple iD username. So if you do have an iPhone, just be aware of this - you may need to change this Apple iD if things persist.
The GDPR regulations mean that giffgaff, even if they could trace the number, cannot tell you who it is.
If the calls are threatening then it is a police mattter. If not then your only option is to change your number.