The southern border of Gad will run from Tamar to the waters of Meribath-kadesh, then along the Brook of Egypt and out to the Great Sea.
Those eighteen thousand measures cannot mean a literal city, no earthly Jerusalem reaches such dimensions. This is the church of Christ in her latter-day glory, spread across the whole earth when the kingdoms of this world become His kingdom. She will stretch from sea to sea, and her name declares her nature: Jehovah Shammah, the Lord dwells there, not in general providence, but in His gracious presence within her.
AI summary
And by the border of Gad, at the south side southward, the border shall be even from Tamar [to] the waters of strife [in] Kadesh, [and] to the river toward the great sea. (f) Which is here taken for Idumea. (g) Which was Jericho the city of palm trees. (h) Meaning, the Nile that runs into the sea called the Mediterranean.
From Tamar - in Kadesh - The former was on the south of the Dead Sea; and the latter, or Kadesh-Barnea, was still farther south, and at the extremity of the portion of Gad, which was the most southern tribe, as Dan was the most northern.