Good Evening 🙂 . Argh... I see your problem :smileysad:. Don't worry, you certainly can get a new giffgaff number.
To do this, please log into your giffgaff account and head to your Profile and Settings details here https://giffgaff.com/profile/details#msisdn. Then, you will see the option to "Get A New Number". Click/tap that option and then follow the on-screen instructions.
Please note that you can only change your giffgaff number once. Any more than that, then a giffgaff agent would need to be contacted (Changing My Number).
For the complete procedure and guide on changing your giffgaff number (including reasons you may have for the change of number and how to resolve issues), these can be viewed here https://community.giffgaff.com/t5/Solve-your-questions/Can-I-change-my-Mobile-Number/ta-p/3488137