Click Layer-New-Layer to create a new blank layer.
Select a new layer, then click the Rectangular Box Selection Tool to draw a rectangular box selection, then set the front background color to white, and click the left mouse button with the Oil Knee Bucket Tool to smear the rectangular box, as shown in the figure below.
Then click Filter-Blur-Gaussian Blur to set the radius to 4.
Then right-click the copy layer-copy 1, repeat the above operation, and set the blur radius to 8.
Then right-click the copy layer-copy 2, repeat the above operation, and set the blur radius to 14.
At this point, the white rectangle seems to be glowing. Press ctrl to select three layers, and then right-click the merged layers.