This is a Biblical name. It is found many times in the Book of Numbers (fourth book of the Five Books of Moses). The first reference is in chapter 1, verse 10, occurring during the listing of the names of the leaders of the tribes. The leader of the tribe of Menasheh was Gamliel, son of Pedatzur.
Note the correct pronunciation: gam-LEE-el or gam-LEE-ayl. There is no 'ah' sound after the 'm'.
The meaning of the name is "G-d has rewarded me".
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