I am not sure if I understood your question, but ForwardDiff.GradientConfig is not used to select dimensions for differentiation, but for selecting things like chunk size:
http://www.juliadiff.org/ForwardDiff.jl/stable/user/advanced.html#Configuring-Chunk-Size-1
If you want derivatives along a subset of the input, use a closure.
If you need more help, please post a self-contained MWE.
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