There are of course many ways to accomplish this in Ps.........here's one way using two methods.
Create the boundary. Use whatever method you prefer. Here I have just used a simple rectangular selection across the "canvas"
Open your Gradient Tool. Set Color. Set the gradient (in the gradient editor) to Foreground to Transparent. On it's own layer create a gradient holding down the shift key. You can make multiple passes.
Open your Brush Tool, set to (approximately) Hardness: 0%, Flow: 10%
On it's own layer, create a brush stroke in the color you desire the full length and height above boundary selection by holding down the shift key, clicking and then dragging from one side to the other.
Add a layer mask to either the gradient layer or the brush stroke layer. Open the Brush Tool. Set the brush to black, brush out/down the ends. This may take several tries and brush setting changes. Start with what's listed above.
Try both methods. I tend to get better and more accurate results with the Brush Tool method.
Playing with this, I found you can also use a reflected gradient rather than a linear gradient:
Set up your gradient tool with the color you want and how you want it to fade out at the end.
Then on a transparent layer, draw your gradient from the center out using the reflected gradient setting. The gradient will be reflected on the opposite space
Then you can apply a mask and soften the bottom or the ends
Reflected gradient tool:
My working space - the blue line is a guideline showing center of document, the arrow on the layers panel points to the mask, the arrow on the tools panel points to the gradient tool: