Mr Gemayel, a leading anti-Syrian politician, was reportedly shot in the street in a Christian suburb and rushed to hospital, where he died.
His death comes amid a political crisis in Lebanon, following the resignation of six pro-Syrian cabinet members.
Mr Gemayel was a member of the Phalange Party and the son of former President Gemayel Amin.
Gunmen opened fire on Mr Gemayel's convoy as it travelled through Sin el-Fil neighbourhood, Reuters news agency quoted security sources as saying.
The news of the killing was confirmed by Saad Hariri, the son of the country's former Prime Minister Rafik Hariri, who was himself killed last year in a huge blast.
A recent UN report implicated Syria in Rafik Hariri's death, in which Syria has denied involvement.
Saad Hariri, who is also the leader of the anti-Syrian coalition in the majority in parliament, has now accused Damascus of being behind the killing of Mr Gemayel as well.
"The Cedar revolution is under attack...Today one of our main believers in a free democratic Lebanon has been killed. We believe the hand of Syria is all over [this]," he told CNN.
Mr Gemayel's death means the Lebanese cabinet is now missing yet another minister.
The six pro-Syrian ministers quit their jobs last week in an attempt to bring down the government after their calls for a greater role in government were rejected.
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