gammastep does not adjust screen backlight to my knowledge. It
desaturates colors on the screen to make it look dimmer, significantly
reducing the effective dynamic range of the monitor (which is already
reduced a bit by the blue light filter).
This is a major hack that I would not want to use myself. For trying
out proper sun-tracking backlight control, I think the best starting
point would just be to fork wlsunset and replace all the wayland code
with backlight updates (/sys/class/backlight/*/brightness for now as
DRM has yet to absorb backlight control), at least for laptop screens.
It should be noted that the required backlight is not really tied to
the phase of the sun, but is rather tied to the amount of ambient
light. Personally I use a small script that runs when I start sway and
every time the lid is opened to set the brightness based on a random
heuristic I made (I was too lazy to use my ambient light sensor), as my
only concern is spontaneous blinding when opening my laptop at night
after using it during the day.