Ruby Kurosawa's (From Love Live!! Sunshine) catchphrase.
It's supposed to be a joke on words because ganbaru (roughly) means 'Do your best' in Japanese and her name is 'Ruby' so it becomes 'ganbaruby' which means 'Do your rubesty'!
Japanese idiom that means "do your best" or "go for it." Often spouted to boost one's self-esteem and encourage them to succeed. Shouted primarily at sporting events and the like.