Does non-linear mean sRGB slope+gamma, sRGB-equivalent gamma (2.2, no slope), or something else?
sRGB is a specific case of non-linear, which is the common and default case. Basically non-linear uses the TRC of the profile. When it turns out the profile is sRGB (default), then non-linear implies sRGB.
the lack of transparency about what’s really happening is no good for those who use GIMP knowing what they do
I’m not sure how it could be more transparent. This is Free Software. Anyone can know exactly what is happening. It can hardly be more transparent.
Of course I understood that you meant “in the GUI”, but I don’t think that GIMP’s dialogs are the right place to teach people about color science.
e.g. in the export dialog GIMP could expose the gamma encoding,
I can’t answer on this. I am far from being the expert on this. Color science is not my area at all, especially since even though we use GIMP professionally, we don’t need to tweak gamma encoding manually when exporting. Feel free to open a feature request if you think this is needed and the devs who know best will be able to answer you. I am personally more in your “dummy/masses” category and content with it.
Though — maybe I am saying something stupid — but isn’t the gamma information set in the profile? If so, that means you can already set this information in GIMP. Why would you want to double the settings with the possibility to set contradictory information? This seems very messy.
But once again, I am not the color expert here. Maybe your proposition makes sense. Our bug tracker is open to everyone and this is where you should discuss new features, not here.