GH is basically a measure of divalent metal ions, mostly Ca++ and Mg++.
What you need is to remove that, and avoid adding it back. As mentioned above, RO water is the common way to go. It wastes some water and takes a while, but then you have very soft water, that needs to be remineralized, up to the desired levels.
I would only go for it if you are 100% sure. It is very straightforward, but does involve some effort.
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