First thing you need to do is select the GRD tool which can be found in the PS toolbox. You can find the types for the tool along the top of the PS application along with the actual colors you will be using and other settings such as opacity etc. Apply them with different types such as as radial / mirror / angle / reflected / linear. Each type has its use. The Photoshop help guide offers more explanation about the particular type and how the colors are mapped in each.
The origin of the linear type can be anywhere, off document as well or you could use guidelines to add the preset at any position. You can use blending modes as well as transparency in the preset to re-apply the preset over and over to the same document or layer. You can extend the length of the preset to the document and beyond. You can also angle the linear preset
You can set the origin as well as different lengths of the preset. Again, you can combine them depending on opacity and blending modes as well as transparency stops in the actual preset
You can control the position of the origin as well as the angle. If you want to avoid that ugly seam then you can always set the stops at the ends to the same color
Reflected creates a mirror of the current preset - though this depends on how it is applied - if you add it from one end (left to right) to the other (or top to bottom) then you will see the mirrored effect otherwise not. Personally, I wish PS would add a mirror feature to the application such as the excellent effect in Affinity Photo. Use guides to quickly find the center (via the view menu). You can also use selections to say create a column of reflected presets - select one half and fill and select the other half and fill again with the preset added to the center of the selection instead of the document
The diamond can be used to create a square effect as well by just holding down the shift and applying it at 45 degrees and not at 0 degrees etc Of course, it can be used at any angle and length of preset as well as any origin point, inside or outside of the document or layer. Again, you can use selections to create an area for one preset and the inverse selection for another
If you combine the types using blending modes such as darken or screen etc, you can create star types, polygons, crosses etc. They are fairly limited in their types compared with applications such as Painter or SumoPaint (that has some amazing fractal effects which can also be combined and manipulated using the plugin effects to create all kinds of amazing overlays and backgrounds etc as well as combined with symmetry settings) or plugins specific for creating color effects such as the ones available on this site via the Andrew's Plugins section but you can certainly combine them to create a wide range of unique preset types. The combination below uses a number of color effects along with the multiply mode as well as difference mode. You can add the result to a layer and then duplicate that layer and use it again and again. You can also drag the result to the CC libraries and have a folder of colorful objects that can be quickly accessed again and again in your projects and perhaps also use in tools such as After Effects if you use them as part of the Creative cloud. You can use all the diamond / linear etc in a number of ways throughout the application such as in layer styles and you can also now use the multiple overlay feature in the layer styles to create truly amazing colorful styles that can be used again and again with layers as well as text / type layers. Any number of amazing color artworks can be created for use with the presets in combination. They can be used to create unique skylines, backgrounds, overlays, type effects and more
You may find the Photoshop gradients of use and interest, you can use them in all the versions of PS from version 6 as well as all versions of PS Elements from version 1 and also on the PC and MAC