Gabriel "Biel" Rainho was born on December 28, 1985, in Rio de Janeiro, Brazil, into a family that recognized the value of sports in shaping character and discipline. At six years old, he stepped onto the mats alongside his cousins Vinicius Marinho and Felipe Tavares, all three guided by parents who understood martial arts as both a physical and educational pursuit.
His first instructor, Laerte Barcelos, laid the technical and philosophical groundwork for Rainho's grappling development, ultimately awarding him his blue belt. The early investment paid immediate dividends on the competition circuit: Rainho captured five consecutive Brazilian National Junior titles between 1996 and 2000, while also claiming multiple Rio State Junior Division titles beginning in 1994 — a résumé that established him as one of the most decorated young competitors in the country.