Indexing gradient in photoshop?

Hey there,

I'm looking for a solution to being able to index gradients in photoshop so that it evenly indexes the gradient in a pixeled way. At the moment if I select "index colours" it does apply the selected amount of colours to the design but the gradient appears in a weird colour-way with a sort of strange pattern, not evenly.

What I'm looking for is this (I need it for working with industrial weaving machines):

Thank you very much for any idea and help!