@nadenislamarre I will try to fix the resolution with configgen. If you've an idea what the best resolution then give me some hints.
1. no setup --> looks like first image
2. 640x480 --> looks much better see second picture
3. 320x240 --> this is the native resolution of the GPi LCD, looked like first picture
4. 0x0 ---> This was a failure, too. Looked like pt 1 and 3
So my outfindings so far is that the resolution with 640x480 seems to be the best one for GPi case and GameBoy emulation.