It's definitely a bit convoluted now, so unless I am not seeing an easier path, we have to, from left to right:
Select the new gadient tool, and run a gradient across the image.
Double click the gradient in the layers panel, which opens the Gradient Fill panel.
Double click the gradient in the Gradient Fill panel, which 'finally' opens the Gradient editor.
I started with a simple foreground to background colour gradient, because I wanted as close to a clean state to start with.
To adjust opacity, you need to add a new swatch 'above' the gradient, which is the white square I have highlighted in yellow.
Note that the transparent area is most of the image in the above screen shot. To make the transparent strip more narrow, add another black swatch on the left, and red on the right. The colour swatches go below the gradient, but to adjust opacity, place swatches above the gradient.
OK, that's the new way, but you can do it a lot more easily with the Classic Gradient.
Select the gradient tool, and Classic gradient in the Options bar.
Use the gradients drop down, and select Foreground to Transparent.
Make a new layer
With your first colour selected, drag the gradient from one side of the layer to the other.
Note you can hold the Shift key to force the gradient to be horiziontal.
Now select the other colour and sweep a gradient the other way.
Foreground to Transparent allows you to continuously add to the layer, where as foreground to background overwrites the previous gradient.
For instance:
The new gradient tool is powerful, but definitely clunky.
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