There is https://www.ddcutil.com/ It has a command line utility and a qt GUI. Unfortunately the GUI is a bit convoluted, because it exposes every option that you can change on the monitor. I once almost bricked my LG monitor, because I accidentally locked the hardware buttons on my monitor, via an undocumented manufacturer specific option.

For gnome there is https://extensions.gnome.org/extension/2645/brightness-contr... This addon adds brightness sliders in the power menu for your external monitors.