Sorry to dig this old one up again. Shift-r was not working. Then I found this on launchpadhttps://answers.launchpad.net/inkscape/+question/164882
"> Shift + R doesnt seem to be working
It does work, but only in the Gradient Tool context:
1) select the object with the radial gradient
2) switch to the gradient tool (keyboard shortcut: 'g')
3) select one of the gradient stops of the radial gradient
4) apply 'Shift+R' to reverse the radial gradient
I had to close the Gradient editor and just have the Gradient "Tool" active for it to work. I didnt need to select one of the stops like in step 3.
Anyway, regarding preferring mouse or keyboard, if you need the command once or twice a year, a clearly marked button directly on the Gradient Editor UI, is much better for discover-ability, making the software easier to use. And put a pop-up tool tip with "shift-r" on the button, and learn-ability will increase too. All basic ergonomic stuff for both keyboard or mouse users.