Playing a G whistle is identical to playing a D whistle, but the tune comes out in a different key. (Just like playing a guitar with a capo, you play the same chords)

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Anyway, my suggestion is to get any whistle tutor, and play it as if you were playing a D whistle...granted instead of a scale being d,e,f#,g,a,b,c#,d, it will be g,a,b,c,d,e,f#,G, but unless you're playing WITH other people, it scarcely matters.