If you wish to play cross harp in the key of D, you do indeed want a G harp (IV chord to D). But if you have a G harp and are asking what key you're in when playing it crossed, the answer is D (V chord to G). It all depends on which way one asks the question.

I'm of the opinion that an accomplished harp player (definitely not me) can play a chromatic scale up and down on a ten-hole diatonic harp by deftly employing “bent” notes.

      - Mark