Simon, I rebuilt gimp (the whole thing, including gegl but not babl).
The tiff-gamma problem persists.
Initially I assumed that the gamma problem only affected images with
linear gamma color profiles. But apparently it affects all tiffs,
regardless of the embedded color profile. So perhaps it has nothing to
do with Gimp 2.9 per se?
Gimp 2.9 correctly opens 8-bit tiffs. And Gimp 2.8 correctly opens
8-bit and 16-bit tifs, in the process converting 16-bit tifs to
8-bits, as expected.
Anyway, in case it might be useful, I posted a screen-shot and links
to some sample tifs and icc profiles:
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