You question is absolutely legitimate, and far from being simple... to answer, let me fitrst opena little parenthesis to explain what happens when colors are out of the gamut of the destination colorspace.
Let's suppose that the in-gamut pixel values are normalized to the [0..1] range. If you convert from a large-gamut colorspace to a small-gamut one, highly saturated colors that are outside of the gamut of the destination colorspace will have at least one of the RGB channels that is either larger than 1 or smaller than 0.
If the conversion is performed in integer precision, the out-of-gamut colors channels be clipped, and this will introduce a "color distortion" in the form of a shift in hue and/or saturation.
The same happens if the conversion is performed in floating-point precision, but the output is clipped to the [0..1] range.
On the other hand, if the conversions are performed in unbounded floating-point mode, the channel values smaller than 0 or larger than 1 will be preserved, and the out-of-gamut colors will not be "distorted".
For example, if you take an highly saturated yellow in Rec.2020 and you convert it to sRGB, the blue channel will have a negative value. However, if the color is converted back to Rec.2020 in unbounded mode (without clipping), the original highly saturated yellow will be preserved.
To summarize, as far as only colorspace conversion are considered and if they are performed in unbounded mode, smaller gamuts are not a real problem.
The real issue is that we are not just doing colorspace conversions. Once we are in the small-gamut sRGB colorspace, G'MIC will perform complex manipulations of the RGB values, to achieve various types of color edits. And that's where channel values below 0 or above 1 can become a problem... for example when multiplying RGB values, or when computing the Luminance that might become negative as well.
If you want to dig further on this topic (and I highly recommend you to do so if you want to leverage all the potential offered by the CCE GIMP version), I suggest you to look into some of @Elle's articles, for example:
Edit: Elle pointed me to another very useful article that explains, among other things, the terminology used to define colorspaces and color profiles: http://colour-science.org/posts/the-importance-of-terminology-and-srgb-uncertainty/