You can transfer your existing account, including your number, any credit, active and queued goodybags and any payback points, from one sim to another unactivated sim using the sim swap feature.
Link to how to sim swap - http://community.giffgaff.com/t5/giffgaff-Top-Tips/How-to-use-SIM-Swap/td-p/6885194
Link to where you sim swap - https://giffgaff.com/activate-sim
If you're trying to transfer your number to another activated sim on another giffgaff account, then this is not possible. You can only transfer numbers in or out of giffgaff accounts to or from other service providers, not between giffgaff accounts.