After Lavan named the pile 'Yegar Sahadusa,' why does the Torah see fit to add that after, he called it 'Gal'ed?
Ramban and Seforno: To teach us that he conceded to Yaakov's wish that it should be called by its name in Leshon ha'Kodesh. 1
Tosfos ha'Shalem: Refer to 31:47:1:1.
Malbim (to 31:48,49), Ha'amek Davar: Lavan said that the testimony of the rock-pile is a mere witness for today, and not lasting testimony. Tomorrow we will be in our houses, and I cannot harm you. However, the testimony of the Mitzpah (that) you not pain my daughters, is only for the future - "Ki Nisaser Ish me'Re'ehu" (31:49).
In what sense was the pile a witness?
Rosh, Da'as Zekenim (to 31:52), Hadar Zekenim (to 31:52): They inserted a sword in the pile, and said that one who transgresses will be stabbed with a sword and stumble on the Matzevah. Bil'am, a descendant of Lavan, crossed with intent to curse Yisrael. His donkey crushed his leg on the Matzevah (see Targum Yonasan to Bamidbar 22:24), and later he was killed through the sword. 1 "The witnesses' hand will be first [to stone]" (Devarim 17:7).
Tosfos ha'Shalem (1, from citing Pirkei d'R. Eliezer 36): David could not cross to conquer Aram until he broke the Matzevah [even though Aram had already transgressed the Bris twice - Midrash Tehilim 60:1].
Malbim (to 31:45): If a Bris is about something revealed, people can be witnesses. If it is about something hidden, only Hashem can be a witness. They made a Matzevah, which was Kadosh in those days, to be a reminder of the Bris.
Moshav Zekenim: Rashi gave a rule, that whenever there is an Ed for a Bris, punishment is through the Ed. Therefore, Rashi explained (Bamidbar 22:24) that Bil'am's leg was crushed against a stone wall. He was killed be'Charev (Bamidbar 31:8), i.e. a sword known from before (also see Rosh to Bamidbar 31:8). Refer to Bamidbar 24:21:151:2 and the notes there.