I am sorry to inform you that once a SIM card is disconnected for the network, you would lose any credit you had on that SIM card (and the mobile number will be recycled too).
Sorry to be the bearer of bad news @leomartinus :smileysad:
Taken from Giffgaff Terms and Conditions https://giffgaff.com/terms#ProvisionofService
Our rights to bar or disconnect your SIM Card
13.3. Your SIM Card will be disconnected unless, at least once every six months, your Account is used to:a) make at least one call, SMS or MMS to another number (not including calls to emergency services or Member Services);b) make at least one connection to the Internet;c) make at least 4 received calls of more than 10 seconds;d) make at least one airtime credit or Bundle purchase; ore) receive Payback Points.
13.4. If your SIM Card is disconnected, you will lose any remaining credit or Bundle balance on your Account and if you wish to be re-connected, you will be given a new mobile phone number.