Don't know about the GP, but I have an iRex DR1000 (http://en.wikipedia.org/wiki/Digital_Reader_1000) which was perfectly sized for reading A4/letter papers. But it is bulky and has no wireless connectivity.

I have an Onyx BOOX M92: https://onyx-boox.com/productsinfo/onyx-boox-m92-black-pearl... It did cost 300€/420$ a year and a half ago. The alternative at that time was the Pocketbook Pro 912 (http://www.pocketbook-int.com/de/products/pocketbook-912) at around the same pricepoint, but that seemed less open (technically both run Linux, but Pocketbook didn't publish their modified sources).