Yes, the most common whistle key is D; it is also possible to get whistles in many other keys (most commonly C, Eb, F, G, A, and Bb, though I believe Susato makes whistles in every key). As Bradford says, whistles play well in the fourth of the "main" key. Therefore, a D whistle also plays in G, a C in F, an Eb in Ab, and so on. You only need a few whistles to be able to play in most of the common keys, though whistle players tend to be as prone to WhOA as uke players are to UAS.
Irish flutes are much easier to get in D than any other key, though there are people who make relatively cheap PVC flutes in all the common keys. The more expensive flutes are made with at least a few silver keys (not at all on the same system as the Boehm flute); they allow players more versatility.
Long story short: if you're playing in G, a D whistle will probably be fine. I often prefer to play songs in G on the D whistle because it lets me dip down to the lower fifth.