You can apply them to multiple layers and then combine them with different blending modes. You can combine two or more presets in 100s of different ways by using the layers with different blending mode combinations. Set the layer blending mode for the Photoshop gradient layer via the layer panel. You can also use fill layers and the presets and blend those to combine many times. Below you can see the use of transaprency to create a very limited vocabulary of letters such as perhaps an ident etc. Check out this part of the site if you want a large selection of Photoshop gradients
How to use Photoshop gradients in combination as a layer
You can combine them in millions of different ways creating extreme color designs as well as subtle color effects depending on the blending modes applied. You can also then extended the color designs by using various Photoshop effects. You can then continue to combine multiple presets with that. Below you can see the use of the 'darken' blending mode to create intense line / shadowy structure
How to use Photoshop gradients multiple times using the toolbar
If they contain transparency then you can combine them without resorting to blending modes or layer blending modes and you can then lay them out in numerous ways to create frames, linear effects, radial, button effects and much more. Below you can see a very basic preset added with a dark shadow transparency in radial mode in PS.
You can use channels to create some unique colorful blends. Select the red channel and then add a GRD color effect. Select a different channel in the channel panel and then add a different preset GRD and then go to the blue channel and select a different GRD color. You can also use the same selected preset and perhaps use different origin points and types and lengths (if available to that type). You can then put the channel back to RGB or CMYK. Below, RGB was used as the basis for the image and in each channel a different preset was used with different lengths and origin points.
How to use them with different blending modes
you do not need to use the toolbar at all, you can also add the presets using the layer menu and the new fill content. You can create multiple layers with fill content and again you can set the blending mode to difference and darken etc to blend the fill content in different ways. Below, different presets were used and different angles were used and the top level preset had transaprency so you can see the underlying preset.
All kinds of weird Photoshop gradients and backgrounds can be created
You can also blend multiple different presets by using the new gradient overlays feature in CC 2015 where you can add more than one overlay / layer effect to any layer. You can select a layer and go to the layer menu and styles and then add an overlay and then click the '+' to add another and another and change the preset or perhaps change the origin position and length and change the blending modes (if no transaprency present)
You can combine the multiple Photoshop gradients using the toolbar and the result can then be manipulated using adjustment layers such as black and white adjustment
You can combine multiple layers with different blending modes as well as mixing Photoshop gradients with effects such as twirls
Different blending modes can be used with multiple layers such as photoshop gradients and 'lighten'. Lighten and screen are probably my favourites and most over used of blending modes.
Above, all kinds of Photoshop gradient combinations can be used as the layer styles can be now used multiple times with the most recent releases of CC and can be blended in 1000s of ways using the blending modes within the layer style panel. You can also combine them with color overlays and patterns as well
Above, You can combine the Photoshop gradients using blending modes but you can also use the layers and combine them with transforms to blend distortions of the presets. Literally billions of designs can be created using the presets with combinations of multiple layers
Combining the Photoshop gradient tool presets using lighten. All kinds of weird and wonderful structures can be created