Its sounds like you performed a sim swap, the sim swap process transfers your current number and any active goodybag or airtime over to the new sim, if you want to have your number changed you will need to ask a giffgaff agent to contact a agent go to your My giffgaff page and you'll find the "Ask an Agent" link half way down the left hand side. This is where you will also see when an agent has replied to you. Here's a link https://giffgaff.com/support/ask/call_barring it can take up two 24 hours for a agent to reply to your question.
Sounds like what you've done is a sim swap. This will keep your current number.
To change your number what you need to do is contact an agent and request a number change. https://giffgaff.com/support/ask And at the same time ask for a refund on the credit you paid for but didn't receive.