2.1.5 [W3C-PNG] Section 18.104.22.168, gAMA Image gamma
The specification states:
For gAMA these are the reference viewing conditions of the sRGB specification [IEC-RGB], which are based on ISO 3664 [ISO-3664].
All Document Modes (All Versions)
When the gAMA chunk is present, the gamma conversion to a display gamma of 2.2 is incorrectly applied; a display gamma of ~1.9 is used instead of a 2.2 display gamma.