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As far as I know it is only the water which is needed for a ghusl, I have never heard of soap being mentioned. If one wants to use one that should be fine, but it isn't a requirement.
It doesnt come in the three faraaid of ghusl, or the sunnats.
But I'm not a scholar, this is just what it seems to me as I have never heard soap being mentioned. Inshaa Allah someone more knowledgeable will give a definite answer.