Gichard went out in 29.41s and held off the Canadian Danielle Hanus by 2/100ths.
Teammate and defending world 50m backstroke champion, Auckland's Gabrielle Fa'amausili, 15, set the pace to turn second at the 50m mark, but had to be content with seventh.
Fa'amausili, coached by Igor Polianski at the United club, will be encouraged for her 50m backstroke later in the championships after showing outstanding speed in the first 50m. She clocked 1:00.74, a personal best and under the national age record she set in the heats.
"I am happy to get another PB. I feel better swimming that race now, more confident. I am happy with the outcome and really proud of Bobbi as well," Fa'amausili said.
"My focus is to keep getting PBs and to focus on swimming my races and not worry about the opposition."
Australia's Minna Atherton won the final in 59.58, breaking her own world junior record set in the semifinal.