It is working for me. It opens a page to Dynamic range compression - Wikipedia
But I recall that some things in our non-Flatpak Linux builds may lack full desktop integration that affects other things as well such as Show In Folder. I will simply remove that link after the documentation is done.
Gain reduction is a graph and and not interactive. It verified it is working but you are not doing anything that causes it to reduce. Try this: add a Gain filter before it and crank it all of the way up.
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Ok, I got it to work, with and without the gain filter.
I put the link in the doc (at the bottom), it can be VERY useful 
Thank you, have a great day!