Slash replaced Tracii Guns as Guns N’ Roses’ lead guitarist. His arrival helped establish the classic lineup that recorded the band’s breakthrough material.
Guns N’ Roses formed in 1985 through the merger of members associated with Hollywood Rose and L.A. Guns. Tracii Guns was part of the earliest lineup, but he left after disagreements connected with the band’s direction and touring plans. Slash, a former member of Hollywood Rose, then joined the group.
Slash was not the only early replacement: drummer Rob Gardner departed and was replaced by Steven Adler, while bassist Ole Beich was replaced by Duff McKagan. Together with Axl Rose and Izzy Stradlin, Slash, Adler, and McKagan became the familiar classic-era lineup.
The wording can cause confusion because Slash later left Guns N’ Roses himself, in 1996, and was replaced by Robin Finck during the Chinese Democracy era. That later change does not alter the earlier answer: when the question concerns Tracii Guns’ replacement, the canonical answer is Slash.