I read the article. I might be wrong, but based on personal experience, I believe that G'MIC GUI makes the assumption that the input file is sRGB and in the range of 0-255 because the default GIMP does that. However, I haven't had problems with linearity or posterization because internally, G'MIC doesn't make any assumptions and works in 32-bit floating point precision.
So, I doubt that you need to modify the code. To avoid confusion, I tend to use command line G'MIC. But if you are curious about how G'MIC GUI handles values and color, be sure to check out https://raw.githubusercontent.com/dtschump/gmic/master/src/gmic_stdlib.gmic.