The 34-year-old Maronite Christian was taken to hospital but died of his injuries.
Syria , which has dominated Lebanon for decades, has denied any involvement in the killing. But it has been linked to the murder last year of then Lebanese prime minister Rafik Hariri,
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The shooting comes at a time when Lebanese political and sectarian tensions are already at fever pitch.
Mr Gemayel was a member of the Phalange party and supporter of the anti-Syrian parliamentary majority, which is locked in a power struggle with pro-Syrian factions led by Hizbollah.
Despite his campaign against its interference in Lebanon, Syria described the assassination as a "despicable crime".
However, Saad Hariri, son of the murdered prime minister, said of the latest assassination: "We believe the hand of Syria is all over the place."
There has been international condemnation of Mr Gemayel's murder.
President George Bush said the assassination represented "the vicious face of those who oppose freedom".
He said Syria and Iran were trying to undermine the democratically elected government of Prime Minister Fouad Siniora.
Tony Blair said it underlined the need for urgent work on a lasting peace settlement in the Middle East.
Hours after the murder the UN Security Council approved the setting up of an international court to try suspects in another high-profile political murder in Lebanon.
Then prime minister Rafik Hariri was assassinated in February 2005. It has been suggested that Syria was involved in the murder.
The Security Council also "unequivocally" condemned the assassination of Mr Gemayel.