Last night I made up some Jynnan Tonnyx, from a flavor concentrate I made about a month ago. The concentrate was citrus zest, chinchona bark (thing that makes tonic, tonicy), allspice, juniper berries, and a some stevia. It needs steeping, but the initial taste was pretty damn good. Don't know that it really would pass as a 'gin and tonic flavor', but as the Guide says:

It is a curious fact, and one to which no one knows quite how much importance to attach, that something like 85 percent of all known worlds in the Galaxy, be they primitive or highly advance, have invented a drink called jynnan tonnyx, or gee-N-N-T'N-ix, or jinond-o-nicks, or any one of a thousand or more variations on the same phonetic theme. The drinks themselves are not the same, and vary between the Silolvian "chinanto/mnigs" which is ordinary water served at a slighty above room temperature, and the Gagrakackan "tzin-anthony-ks" which kills cows at a hundred paces; and in fact the one common factor between all of them, beyond the fact that the names sound the same, is that they were all invented and named before the worlds concerned made contact with any other worlds.