If you want to top up at a PayPoint outlet you can buy GiffGaff vouchers directly over the counter, just as you can any other voucher. Ask the store assistant to check on their PayPoint terminal from the list of mobile operators that their terminal supports. It will be listed as just Giff.
The beauty about using a GiffGaff voucher is your credit is added immediately.
Also you can use O2 and Tesco vouchers, if you must!!