There is a possible workaround for this though,
You can't top up with paypal on the giffgaff site BUT you can buy an o2 top up voucher via paypal.
There are plenty of online companies that will email (or text) you a voucher code to top up with for the main phone companies (o2, orange etc.) and most of these voucher companies accept paypal.
Seeing as it seems that o2 vouchers can be used with giffgaff, perhaps this would be an acceptable solution for you?
I can't vouch for this company as i've yet to use them... http://www.topupsdirect.com/index.html
but they are one of many who offer such a service & accept paypal as payment.
But if you google "paypal o2 top up" i'm sure you can find plenty more who do it... there are a lot of ebay sellers who do the same too.
Hope that helps.