‘ And the children of Reuben, and the children of Gad, called the altar [Ed], for ‘it is a witness between us that YHWH is God'.'
The actual name is not in the Massoretic Text. However it is found in some Hebrew manuscripts and in the Peshitta (the Syriac version). We could translate ‘named the altar' (as LXX) but the explanatory phrase following it anticipates a name having been given. Thus the name Ed, meaning witness, is possibly to be inserted although it was not in the text used for the LXX which has ‘And Joshua gave a name to the altar of the children of Reuben, and the children of Gad, and the half tribe of Manasseh, and said, It is a testimony in the midst of them, that the Lord is their God.'
Whichever is correct it is clear that the altar was given a name that indicated that it was a witness between His people on the east of Jordan, and those on the west, that they recognised YHWH as their only God.