I doubt this will help you but I’ll share my experience anyway.
I didn’t know of this addon until I read this topic, but having done it prompted me to read the original topic about it (Add a "Recent" sub-menu), and having done that, I tried to install it on my Windows Gramps v6.0.4.
GitHub is a deep mystery to me but I found the relevant folder:
and RIGHT clicked on each of the .py files and saved them to my Downloads folder. However, when I copied them to a subfolder in my Gramps plugin folder, and restarted Gramps, the addon didn’t work.
I went back to GitHub and, in turn, selected each of the .py files and used the ‘download’ raw files option. Using these versions, the addon now works.