Numerous Aharonim refer to לשונו הזהב של רש"י; Rashi's golden words (or golden expressions). These include, for example, the Sha'ar Ephraim (46), R. David Pardo (Mikhtam L'David to YD 32), Hida (Shu"t Hayyyim Sha'al 2:41), R. Elazar Landau (Yad HaMelekh to Hilkhot Sotah 1:15), Shu"t Ben Yamin (41), R. Meir Simha of Dvinsk in multiple places such as Ohr Sameah (to Hilkhot Sehehita 12:3), the Seridei Esh (Makkot 48), and numerous others.
Importantly, however, this expression is used about other rabbis such as Rif (cf. Rabbi Akiva Eiger to Pesahim 30b).
Much more significant, in my opinion, would be to look various rabbinic descriptions of Rashi's commentary, not limiting it those that happen to compare it to gold. Would those comparing it to gems, for example be any less meaningful? Or would those actually describing the greatness of his work directly; without metaphor, be any less significant?
One such example is the Meiri's comment in his introduction to his commentary to Avot that
וראש לכל החיבורים שנתחברו דרך פירוש הם פי' רש"י ז"ל ואם רבו הלוחמים עליו כלי זיינו עליו ותשובתו מתוך דבריו כולם נכונים למבין אין מעלתו נכרת רק ליחידים כי במלה אחת יכלול לפעמים תירוצים של חבילי קושיות
At the head of all works composed as explanatory commentaries is the commentary of Rashi. And although his opponents are numerous, he is equipped with his weapons, and his responses are implicit in his words - they are all proper to the understanding. His greatness is only evident to individuals, for in one word he sometimes includes solutions to loads of questions. (Translation my own).