Just thoughts- Is it possible the switch at the base of the panel is 'off' - but that would not explain the effect visible on thumbnails!
What color is your overlay? (Shift+O) if it is white, then exposure effects may not be visible until overlay is off (O)
A universal answer might be to reset the Preferences file. Resetting Preferences in Lightroom CC/6
Thanks for the reply. It looks like I somehow had the overlay color set to white for the gradient tool (have no idea how in the world I did that) Problem seems to have been fixed now. Thank you.
So Michael- what was the correct answer? What did you do to see the gradient? (for the benefit of others )