maybe your grandfather meant the Umatilla Indian Reservation, in eastern Oregon. The reservation is located near the city of Pendleton on the north side of the Blue Mountains. Home of the Cayuse, Umatilla and Walla Walla.
I ran a check and did not come up with a Gambrell on any reservation and if it is Umatilla there is no one with this last name there. Do you have any other information that might help like a frist name, since there is no Blue Mountain reservation in United States.