Rashi: [Even] if you were dear to me like the Beis ha'Mikdash, which is high on top of a mountain, [would I not make you like a Midbar, i.e. desolate]. The Mikdash is called Gil'ad, for from it is sap ¹ and a cure for the entire world.

Radak: He says about Beis Melech Yehudah, which is the head of Mamleches Yisrael, you were to me like Gil'ad. Gil'ad was a city in Ever ha'Yarden, and that entire land of Sichon and Og and the entire Bashan was called Gil'ad due to it, for it was the capitol. So you were to Me, the head of Mamleches Yisrael, for Tziyon and Yerushalayim is the head of all Eretz Yisrael. Now I will make you like a Midbar, and I will make the cities around you as if they were not inhabited.

Malbim: Even if you would be to me like Gil'ad, an inhabited land

Radak: You were also the head of Levanon. Eretz Yisrael is also called Levanon - "ha'Har ha'Tov ha'Zeh veha'Levanon" (Devarim 3:25).

Malbim: It is a land full of Arzei Kel (great cedars) and all trees.

Radak: This is an expression of a Shevu'ah - "Im Lo Shivisi v'Domamti" (Tehilim 131:2), "Im Lo Batim Rabim" (Yeshayah 5:9) and similar verses.

Radak: The writing refers to ha'Aretz as a Klal, and the pronunciation refers to the cities.