While @Tobias is correct on one level, many non-builtin G’MIC commands (builtin are denoted by “(+)”), and almost all of the GUI filters, assume that your input files are sRGB or at least RGB (which in fact are sRGB linear) in the [0,255] range. When in doubt, refer to
- http://gmic.eu/reference.shtml
- https://raw.githubusercontent.com/dtschump/gmic/master/src/gmic_stdlib.gmic
- gmic-community/include at master · dtschump/gmic-community · GitHub
Or simply keep it in regular sRGB or linear sRGB and between 0-255.
That said, it is possible for you to write your own colour conversion commands. @David_Tschumperle has exposed the existing colour conversion commands in gmic_stdlib.gmic to demonstrate how it could be done. You just need the right mathematics for your particular set of conversions.