In Biur Hagra YD 293:2 the Gra makes the following shocking statement:
ודברי בה"ג כאן טעות גדול הוא וא"צ להשיב עליהן וגם מ"ש בשם ב"ח הב"ח כשיטתו דס"ל אינו נוהג בשל עובדי כוכבים וכבר הכו על קדקדו כל האחרונים ושגגה יצאה מתחת ידו וטובה ה"ל לבה"ג כאן השתיקה
And the words of Bahag here are a great error, and there is no need to reply to them, and that which he wrote in the name of the Bach, the Bach is simply going according to his own opinion that it is not observed when it comes from a non-Jew, and all of the Acharonim have already hit him over the head, and a mistake has come out from under his hand, and it would have been better if Bahag would have been silent. (my own very loose translation)
How could the Gra attack the Bahag so harshly? Isn't the Bahag (The Baal Halachos Gedolos) a Gaon who lived close to a thousand years before the Gra. Why would he talk so sharply against such a renowned authority?