TPS is not specifically a giffgaff service – it’s for everybody (landlines or mobiles).
It’s free – provided you get it from https://www.tpsonline.org.uk/register
But there are probably still cowboy companies that claim to offer a “service” to register you for TPS. Those services charge, and they may or may not actually register you, and they may or may not use your information for improper purposes. So make sure you go direct to the TPS site, not through one of the cowboys.