Why do Gargantians call Chamber "Yunboroid" or "Yunboro"?
The correct romanization is probably "Yumboroid." The word is probably a portmanteau of "yumbo" and "droid." 

Yumbo was a French trademark name of excavators, now used in Japanese to refer to excavators in general (much like dumpsters in America, which was the trademark name for a mobile garbage bin).