Quote:Yes but why over the whole image?
Because that is the image / layer size and your pasted in alpha channel has lots of semi-transparency.
This is a dark to light gradient in the alpha channel, no 100% black for fully transparent, no 100% white for full opacity just semi-transparency all over.
Going back to your dislike of the checker pattern, change it to either white / black / grey in Gimp preferences.
Same image, black representation of transparency.