Fernandinho and Luan scored in the first half as Gremio triumphed 3-1 on aggregate in the South American showpiece.
It was Gremio's first Libertadores title since 1995, ending a 22-year wait for the Brazilian side, which also booked its place at next month's Club World Cup in the United Arab Emirates.
Gremio carried a 1-0 lead into the midweek return fixture thanks to Cicero's 83rd-minute strike last week.
The Brazilian club continued where it left off away to Argentina's Lanus courtesy of Fernandinho.
Fernandinho increased Gremio's advantage on aggregate with a powerful finish in the 27th minute at Estadio Ciudad de Lanus – Nestor Diaz Perez.
Luan silenced the home crowd three minutes before half-time, when he weaved his way past three defenders and scooped the ball over Lanus goalkeeper Esteban Andrada.
Jose Sand's successful penalty pulled a goal back for Lanus with 18 minutes remaining but it was nothing more than a consolation as Gremio – who finished with 10 men following a second yellow card for Ramiro - now look ahead to a Club World Cup semi-final showdown with either Pachuca or Wydad Casablanca on 13 December (AEDT).