You can find it via the layer menu and fill layer and gradient.
On using the command, a panel appears with blending mode / name etc (it is a pity that is not on the actual fill layer panel) so set the blending mode etc, you can always change that later. Press OK. Select a preset, you will see all the presets available and you can always go to the right side menu of the preset picker and select import some more amazing content. Set the style for the current preset such as linear, radial, etc. Set the angle as well as scaling and the other options such as reverse etc. You will see a preview. You can interactively re-position the layer by dragging on the image. Press OK. You can now see the layer with the two square boxes in the layers panel
By default, the entire selected area (or document) is covered by the color. You can mask it so more or less of the colors are show. To show the colors, the mask is white, to hide it, is black.
Click on the thumbnail for that entry (the second square) and then apply brushes, pixel shapes etc as well as effects such as Gaussian blurs, twirls etc
You can also modify the grays by using image menu adjustments such as levels and invert etc though some will not be available such as variance.
Double click the first thumbnail in the entry to change the content. You can then shift the origin, you can change the scaling and angle, style (linear, radial etc), reverse the preset change the preset etc and once you are happy with that, press return.
They are just normal layers, though a layer containing no pixel information, you can combine multiple layers and use blending modes as well as any changes to the mask to create all kinds of combinations of presets. Either create a new fill layer and set a new preset or go to the layer menu and duplicate the existing one and then change the blending mode of the top layer.
You can't just go to the layer and apply an effect, there is nothing there to work with. Workaround is required. Select the layer (s) and then go to the layer menu and smart objects and convert to a smart object and then apply filters to the colorful design so you can add a twirl etc. The beauty is that you can then double click the smart object at any time and then modify the preset and style etc as well as change the origin position and blends etc and then go back to the smart object with your updated design.
You can also selections. Create any selection such as the rectangular marquee tool and define an area smaller than the document. Go to the layer menu as before and create your layer with the preset and you will notice that straight away the area covered by the design this time around is limited by the selection you used. You don't have to deselect, it is no longer being used. Go over to the panel and you will see you now have a mask added and as before, you can manipulate that by adding blurs and twirls etc to modify the pixel information and as you do that, you will see more or less of your preset. White area shows all, black shows nothing, use grays to vary the amount.
With the latest versions, you don't have to use the command to create the layer, you can also simply select a preset from the gradients panel (found in the window menu) and drag there onto the document and you have a new layer. If you drag another one to the same layer it will just replace the existing colors. If you hold down the alt / option key as you drag then a new fill layer will be generated and is a quick way to add multiple layers and you can then change the blending modes and opacity as required