Brisync is described as 'Synchronize your external monitor brightness with built-in display. Application reads current brightness of your built-in display and passes it to external monitors via DDC interface. You can adjust brightness by menu in system status bar' and is an app. There are more than 50 alternatives to Brisync for a variety of platforms, including Windows, Mac, Linux, Android and Wayland apps. The best Brisync alternative is f.lux, which is free. Other great apps like Brisync are Twinkle Tray, Redshift, Solace and LightBulb.
Small GTK GUI application to set contrast, brightness and gamma for wayland compositors which support the wlr-gamma-control protocol extension. Basically this is the example from here: https://github.com/swaywm/wlroots/blob/master/examples/gamma-control.
