[kounapuu]

Do any devices use GAMA or PHYS properties from PNG files?

With a PNG file, you can include two data chunks called GAMA and PHYS.

GAMA is a control for brightness of an image on any display (I guess to allow for brightness consistency across all kinds of displays).

PHYS controls the pixel aspect ratio. Looking at TVs, PC monitors, LCDs, handheld devices... some displays may have round pixels, some square, some rectangular. This PNG chunk is supposed to control how the image looks with different pixel sizes.

Do mobile devices even look for these properties in a PNG file?

These chunks, along with others, are usually stripped out by PNG crushers... but PNG crushers were made thinking that PNG files are only being used for web browsers.