Businessman Adama Barrow of the main opposition United Democratic Party (UDP) beat candidates from three other parties at a convention held in the capital, Banjul.
Mr Barrow, 51, is quoted by AFP as saying:
We have put our differences aside for the interest of this country. Gambians are tired of 22 years of Yahya Jammeh's misrule and will end it come December 1st when we go to the polls. We all have to join hands to save this country from destruction."
The Gambia, a popular tourist destination because of its beaches, has never experienced a democratic transition of power since independence.
Mr Jammeh took over in a 1994 coup - his critics accuse him of running a repressive state, an allegation he denies.