Try to double-click on the word COLOR. That should reset the slider to default.
Thanks for the tip Bob, but unfortunately it didn't work.
Thanks again Bob. I tried option Restore Default Presets before double clicking on the word Color and the grey panel issue is back, though intermittently. Sometimes it'll apply a grey layer over the image and display the dot that indicates the middle of the gradient, while other times it doesn't display the grey layer or the dot.
Oh, I just discovered a couple new issues:
1. Selecting a new image in the film strip changes the navigator thumbnail but no the main canvas in the center.
2. When I tried to delete the gradient dot I created (or attempted to create above) I received the following error (text included for search to find):
Unexpected error performing command:
Which leads me to this post: Lightroom: Problem exporting images when spot removal was used
and subsequently Recover catalog and images after resetting preferences which did work. I removed the preferences file and the gradients now work as expected. The menu options in Lightroom > Edit > Preferences > Presets > Reset all default etc etc... did not reset these preferences successfully so manually removing the files seems to be the solution.