The key thing for the layer in Photoshop is that there is a layer and it has been filled with something, it is no use trying to use a gradient overlay on an empty layer. Go to the layer menu and layer styles command and gradient overlay
So now you have one overlay and you can change the preset via the drop down. You can also set the blending mode to difference, lighten, multiply etc. Set whether it is linear, radial etc and set the values for those.
Click the + button in the gradient overlay line. Shift the origin of the new effect. Choose a new preset from the drop down. Change the blend mode If required (such as difference). You can do this multiple times to create a far more complex style design (you can also combine it with other overlays such as color and patterns)
You can create multiple gradient overlays by using the radial option (say though you could equally use the linear or diamond etc) and setting the scaling to 50% or certainly something less than 100%. You can then shift the overlays around and they end up looking like spheres or colorful bubbles combined (though the result depends on the combination of the multiple presets and the stops used in those)
Duplicate layer with multiple gradient presets via the layer menu duplicate. Change the blending mode via the layer panel to something like difference or lighten etc. Double Click on the entries in either of the layers and change them to linear, change the origin, length etc. You can add many additional layers
When you edit the layer styles with multiple gradient overlays you can always save the result to a style by clicking the new style button in the layer style panel.