Posted by Adam Quinan on February 17, 2014 at 13:21:34 user Adam.
Most people consider that Gibber the monkey's name comes from "gibbering" an notably simian activity and therefore should be pronounced with a soft G (Jibber).
However, given Mrs Walker's Australian connection, perhaps the children had picked up that Australian word "gibber", a stone, which is said with a hard g, like gibbous. It most often appears in gibber plain, a phrase describing extensive stony areas, and is from the Dharuk Aboriginal language of the area around Sydney. (info from Michael Quinion's World Wide Words newsletter)