The answer appears to be: sRGB, the color space to which gimp defaults
(unless you've specified another color space?).
The formula for converting sRGB levels to relative luminance is in the sRGB
spec. A table of the Levels and their corresponding relative luminance
is here: http://www.chaosreigns.com/hdr/srgb/
This is very close to gamma 2.2 (a logarithm?), varying only slightly
for the sake of invertable binary math.
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