Hello,
ArgEditor can open carsets from F1Ed, so you can convert them that way.
The F1Ed carset format is a relatively straightforward text format.
The color of each car and helmet segment is defined from the F1GP color palette.
With regards to the GP Editor, I'm not familiar with the file format.
However, the source code of the GP Editor is publicly available, so the format could be extracted from there:
It would be great if you could make any progress with the conversion.
Cheers,
Hrvoje