Here's one way.......
Sample the two colors used in the gradient on the Horiz stab.
Make a selection like this using the Pen Tool
View attachment 85530
Create a new layer.
Fill the layer with the lighter color of the to colors you sampled earlier.
View attachment 85531
Create a new layer....
Select your Gradient Tool
Set the Gradient Tools preset to foreground to transparent.
Set your foreground color to the darker of the two colors you sampled earlier.
Create a gradient from the bottom to the top while holding the shift key.
(note: you can always backstep if you don't like the gradient)
Hit Cmd/Cntrl + D to deselect.
Done
View attachment 85532
View attachment 85533