Bongo is favored to win a third term at the head of the oil-rich West African state ruled by his family for 55 years.
The 64-year-old, who took over from his father Omar Bongo Ondimba in 2009, officially announced in July that he would run again.
His leading rivals include Alexandre Barro Chambrier of the opposition Rally for the Fatherland and Modernity party and the National Union's head Paulette Missambo.
The opposition failed to agree on a single candidate to challenge Bongo in the August 26 poll, but candidates are former ministers and part of the Alternance 2023 coalition.
Bongo's powerful Gabonese Democratic Party holds strong majorities in both houses of parliament.
The president was narrowly re-elected in 2016, with just 5,500 more votes than rival Jean Ping who claimed the election had been fixed.
Announcement of the results sparked violence in the capital Libreville that left five dead, according to the government. The opposition says 30 people were shot dead by the security forces.
Bongo suffered a stroke in 2018 and spent months on the sidelines recovering, leaving the opposition to question his fitness to run the nation.
In April, the Gabonese parliament voted to amend the constitution and reduce the president's term from seven to five years.
The presidential ballot will coincide with elections for the National Assembly and regional and local councils.
Gabon is one of the richest countries in Africa in terms of per capita GDP due largely to its oil revenue and relatively small population of 2.3 million.
But a third of the population live below the poverty threshold, according to the World Bank.
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