Is there a GUI program/widgets that will allow me to control the screen brightness of the external monitor?
See if this thread can help you… 
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Tnx but neither of them works, I use Kde+wayland and one HDMI external monitor that I used to change the brightness with emoacht/Monitorian easily in windows.
Well, the fact that you’re using Wayland is important information. You could try iris-flower-wayland and/or wob, both from the AUR.
pamac build iris-flower-wayland
pamac build wob
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If your monitor have i2c interface you can use vdu_controls
https://forum.manjaro.org/t/favorite-lesser-known-apps/5481/171?u=tomek
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install digitaltrails/vdu_controls but it show error message: No controllable monitors found, exiting. Run vdu_controls --debug in a console and check for additional messages. Check the requirements for the ddcutil command.