Some (i.e., Benjamin Mazar) claim the Geba in 2 Kings 23:8 is a claim of the maximum extent of the Kingdom of Judah in the reign of King Josiah. However, this is somewhat unlikely. There is no evidence for any Geba outside that of Benjamin in the Bible. 2 Kings 23:8 comes before the verse regarding the destruction of Bethel’s altar and refers only to the purification of the priests of Judah, and seems to refer only to heartland Judah, before any expansion into Mount Ephraim took place. Geba of Benjamin was, indeed, the northeasternmost fort of Judah since the days of Asa (1 Kings 15:22), just as Beersheba was heartland Judah’s southwesternmost fort. Indeed, 2 Kings 23:8 may be paralleled by Zechariah 14:10 (though no one knows quite where Rimmon is).
In 1 Kings 15:16-22, an interesting story of a border conflict is told. According to 2 Chronicles 13, Abjah took Bethel in a conflict with Jeroboam I. This is, however, unlikely, or, if Abjah did take Bethel, he held it for only a limited time period. According to the 1 Kings narrative, Baasha of Israel first fortified Ramah “in order to prevent anyone from going out or coming in to Asa king of Judah”. Was this Ramah er-Ram or, as in Judges 4:5, Ramallah? Geba is surely the present Jaba, and Mizpah is very likely the present Tell el-Nasbeh, on the main road between Bethel and Jerusalem (Nebi Samwil is not on the main road and does not have any Iron I remains to correlate with Samuel). Considering that the fortification of Ramah was meant “to prevent anyone from going out or coming in to Asa”. It was probably er-Ram, south of Geba.