I then bought the Amazon Powerfast usb power adaptor (the one they sell for the Kindle Fire tablet, 1.8A) to keep at my Mum's. It didn't come with a cable, so originally I used the new charger with the Kindle e-reader cable - the G Pad still took twice as long as usual to charge.
Using the new Amazon charger with the cable that came with the G Pad, everything is back to normal.
So it seems like both charger and cable need to be up to the higher power requirements, and all usb charging cables are indeed not created equal - the G Pad one is certainly much thicker, heavier duty looking than the e-reader one. I'm going to pick up Amazon's Powerfast charging cable to go with the new charger now.