Luan gave the visitors the lead at the Monumental stadium in the Ecuadorian port city on Wednesday by deftly lofting a shot over goalkeeper Maximo Banguero after Matias Oyola's hashed clearance, reports Xinhua news agency.
Defender Edilson then powered in a low 30-yard free-kick, before Luan netted his seventh goal of the tournament with a neat first-time finish after Edilson's cross.
The decisive second leg will be played at Gremio Arena in Porto Alegre next Wednesday.
"We still have another game so it's far from over," Luan told reporters after the match. "Their team is fast and has a lot of quality. They didn't make it this far by accident so we need to play another great game in Porto Alegre."
Gremio are bidding for their third Copa Libertadores title and first since 1995 while Barcelona have never won South America's top club competition.
On Tuesday, River Plate secured a 1-0 home victory over Argentine rivals Lanus in the other semi-final first leg.
The winner of the Copa Libertadores will earn the right to play in the FIFA Club World Cup in the United Arab Emirates in December.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
About VDC
Doraiah Chowdary Vundavally is a Software engineer at VTech . He is the news editor of SocialNews.XYZ and Freelance writer-contributes Telugu and English Columns on Films, Politics, and Gossips. He is the primary contributor for South Cinema Section of SocialNews.XYZ. His mission is to help to develop SocialNews.XYZ into a News website that has no bias or judgement towards any.