In parshas pinchas ch 26 vs 29 we find Menasheh had a son Machir who had a son Gilaad (גלעד). In fact this family group was called "Gilaadi" (גלעדי). At the end of parshas Matos ch 32 vs 39 - 40 we are told that the children of Machir went to a PLACE called Gilaad and conquered it. Moshe subsequently gives the Gilaad to Machir's children. Is this just a strange coincidence?
See Rashi, Sifsei Chachamim and Ramba"n on Breishit 31:48-50 who explain a reference to why in Sefer Shoftim, Har Gil'ad is called "Mitzpeh" over there.
In this verse, Rashi makes a reference to the word "Mitzpah" which means "look-out". Rashi on v 49 starts by explaining "The 'Mitzpah' that is in Mt. Gil'ad" prior to referring to the verse in Shoftim. In Shoftim it is pronounced "Gil'ad"; In Breishit, it is pronounced Gal'ed". It seems to be the same place.
I do not know why or how the pronunciation was changed.