After just some steps, before reaching the first blocks, I automatically get the head-wings.
If I jump now, I'm flying up through the top of the sky background, and end up on the top a white background.
I can then fly to the left or to the right, but I can't see anything as I suppose I'm stuck at the top of the level 1 map.
It's very strange, it's as if most Game Boy CPU instructions were working, or I would have gotten garbage, an early freeze / nothing, or a crash... but some CPU instructions were failing.
All files were sent from my CG20 on which they are working to my Graph 90+E.
We can suppose the data transfer is not working properly for some reason between these 2 models, and is storing corrupted files, which could explain why CGDoom complains about a missing doom.wad file.
I'll try again as soon as possible by retransferring the same files, but using the computer this time.