Select the Photoshop gradient tool. Go to the top left and click on the editor on the top left of the control bar. Go to the right side menu of the panel and click the new command. You will now see the editor. You can re-size this panel, probably makes it easier to stretch it out to fill the entire screen.
Select one of the existing preset, they make a great start for a new preset. Select the first entry (most likely listed as Foreground to Background. Set the type, solid or noise. Set the smoothness, probably best to keep it at 100%. If you set it to 0%, you may notice a slight color change. You will see a bar in the middle of the panel. If you go for the noise type, you will see no stops. If you go for the solid type then you will see color stops along the bottom of the bar, opacity stops along the top.
The opacity stops are a between white and black (100% opacity). You can have one or more of these along the bar and it will change how you see through or not with the blend. If you use one at 0% then you will be able to see through the applied effect to the underlying layer or the background. Set to 100% and it is a solid color. You can set it to anything in between. You can move the stops to any position interactively or using the controls
You can have one or more color stops along the bottom of the bar. Set the first stop to green and the bar will go green for a part of the bar until you get the next control. There is a midpoint control between the two but it is only visible when you click on of the colors. You can then modify how much of one color or the other you get via the midpoint. You can interactively change, add, delete the stops by clicking and moving these controls as well as changing the settings via the location etc entries below for each item.
You can create all kinds of amazing artworks using the noise feature but it takes away the interactive controls. Change the roughness to create very dramatic changes to the bar. 0% roughness and you have a smooth design, set to 100%, you will see stripes of color along the bar. You can also change the color model used and as you change that, the sliders below for the individual channels will change. Change the slider settings to set a maximum and minimum for the range to be used in the creation of the stripes. Click add transparency to add random opacity to create gaps between the colors. There is also a randomize button to click to create a random noise preset. Once you are happy with the colors, click the new to add it to your collection
Give it a name, that is in the middle of the panel. Once you are happy with your preset, click new on the middle right. It will be added to the current selected category