Garth is a character in The Testaments.
Despite the law against Texan citizens acting against Gilead, Garth emigrated to Canada and became a Mayday agent. He came into contact with Daisy, the famous Baby Nicole, after her adoptive parents Neil and Melanie were killed by agents of the Eyes in Canada. In order to get Daisy to Gilead, he made her pose as a homeless girl and pretended to be her adopted boyfriend. Garth and "Jade" hung around areas frequented by the Pearl Girls and after a few days, Garth "sold" Jade to the Pearl Girls as part of Mayday's ruse.
In Gilead, Daisy remembered Garth and used his name when she made up the story of "Jade" running off with an Economan plumber.