wasn't sure how to phrase this so I thought I'd post an example of the kind of effect I'm looking for.
I understand that this could simply be made with the gradient tool but not sure how to get the 'spread'
any help would be massively appreciated.
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It looks like it might just be a regular linear gradient that has been masked using a radial gradient.
You can create custom gradient using the gradient editor.
Then over a black layer, I filled a new layer with the Rainbow gradient I made. Then I used a black and white gradient to create a layer mask consisting of a radial gradient, which I stretched into an oval.
Just in case the problematic colored area is the smaller one which contains everything that a rainbow has you can try this:
Have a black background layer. Then you make the colored shape to a new layer. In Photoshop you can draw a path, convert it to selection and and fill the selection with a gradient:
(sorry for irregular looking path, you can get easily better result in Illustrator where working with paths is much more comfortable - copy the result and paste it to Photoshop)
Then apply Gaussian blur:
If the edges seem too thin, undo and try smaller blur radius.
You can also duplicate the layer once or more times and merge the duplicates. It can be a good idea to squeeze the duplicates vertically to avoid the shape becoming too high. It's tried here:
If you merge the gradient shape with black BG before blurring it's possible to restore the gradient fidelity to some degree with curves: