Ask Blaze/Tommo or whosoever produced GG. There's no source code for native os or opendingux kernel, so no way to fix it.
From technical side, jz4755's tv encoder is not able to show stretched 320x240 picture, you get a small 320x240 window on 640x480 screen in fact. And IPU can't help at all because it doesn't understand RGB input (only YUV)
This was resolved in Dingoo a380 with 2 hackish ways: 1) lcd driver upscaled small picture to 640x480 in software 2) the emulators themselves upscaled to 640x480