Using panorama feature sometimes happen i have different tones moving across the picture (eg. because of the sun position). At the end i have this result.
Gradiated mask is just a layer mask fading from black to white reducing the intensity along the fade. Gradient maps are different as they reassign colour based on luminance.
So, for your image I color corrected the left side then made that correction fade out to the right. Then repeated for the other side.
You need to work a little on each individual frame first. Make sure to not only match exposure and white balance, but also vignetting and geometric distortion (the latter two handled by the lens profile).