In a saucepan, boil the Gewürztraminer over moderately high heat until reduced by half, about 10 minutes. Whisk in the butter and remove from the heat.
Using kitchen shears, cut along the length of the crab leg shells. Arrange the legs in a single layer in a large steamer basket or bamboo steamer; steam until heated through, about 4 minutes. Transfer the crab legs to a platter.
Gently rewarm the sauce over low heat and stir in the chives, lemon balm and spearmint. Pour the sauce into warmed dipping bowls and serve at once with the crab.
An off-dry, spicy Gewürztraminer will echo the flavors of the sauce and complement the sweetness of the crab.