Gabigol scored twice at the Maracana in Rio de Janeiro, where Flamengo cruised to a 6-1 aggregate triumph on Wednesday.
Flamengo will now face defending champion River Plate in the Libertadores final in Santiago on 24 November.
The semi-final tie was deadlocked at 1-1 after Flamengo's draw at Gremio in the opening leg on 3 October, but Jorge Jesus' men blew the match wide open after half-time.
After Bruno Henrique broke the deadlock in the 42nd minute, Gabigol doubled the lead with a thunderous half-volley into the roof of the net within a minute of the second-half restart.
Gabigol – on loan from Serie A giants Inter – made it 3-0 after converting a 56th-minute penalty before Pablo Mari headed past Paulo Victor 11 minutes later.
Flamengo – champion in 1981 – showed no mercy as Rodrigo Caio scored the team's fifth goal via a glancing header 19 minutes from the end.