Because them's the rules! :-)
Attributive adjectives agree with their nouns in case, number and gender. Gruaig is a feminine noun, whether it occurs in a place that triggers lenition or not, so the feminine form of the adjective is always used after gruaig, even if gruaig itself isn't lenited. Remember that many feminine words start with a letter that can't be lenited, even after an, so, for example, you say an lámh chlé for "the left hand", because lámh is feminine, even though it can't be lenited.