Voting started at 0800 GMT with dozens already queuing at roadside polling stations to be among the first to cast their ballots.
Voting started at 8am (0800 GMT) with dozens already queuing at roadside polling stations to be among the first to cast their ballots. More than 880,000 Gambians are eligible to vote, with polls open until 5pm.
"Things are equal, that's why you have a lot of participants," said Ousman Manneh, 34, an assistant returning officer at a polling station in Bakau, a town not far from The Gambia's capital.
The first results are expected during the evening and a full set due by late morning on Friday.
The poll is a key test for several former opposition parties that united to form a coalition in December to oust Jammeh from power and deliver flagbearer Adama Barrow to victory as the new president.
Internal tensions mean those parties are not running together in Thursday's legislative polls.
As a result nine parties are in contention, including Jammeh's Alliance for Patriotic Reorientation and Construction (APRC) and the strongest traditional opposition force, the United Democratic Party (UDP).
They face a significant threat from the Gambia Democratic Congress (GDC), a youth-led party which did not join the governing coalition, and whose leader Mama Kandeh came third in the presidential poll.
There is no formal opinion polling in the tiny west African nation, making it difficult to establish voting intentions, but the scarcity of APRC rallies which once dominated every street during campaigning suggests voters are unlikely to give them the near-total majority seen in past elections.
Anna Stelle, a 69-year-old former APRC supporter, said she had switched to the GDC, along with many other former Jammeh supporters.
"I voted GDC," she told AFP. "I will never vote for APRC again and the other is UDP, I am not comfortable with them," she added, calling the UDP "tribalist" troublemakers.
There are 53 seats up for grabs in The Gambia's National Assembly, five more than in 2012, according to the Independent Electoral Commission (IEC).
Five extra places are appointed by the president to give a total of 58 seats in the legislative chamber, which was long derided as a rubber stamp for Jammeh's executive orders.
Alagie Bubacarr Jallow, an unemployed hotel worker, said his town had long been ignored by the Jammeh regime because it was seen as an opposition stronghold.
"We need things to be done in Bakau," he told AFP. "I'm voting for Assan Touray, UDP, but more for the candidate. He is loyal, ambitious and respectful of all the people. He can try and fight for the needs of the town."
The African Union, regional bloc ECOWAS and the European Union have all sent observers to monitor voters casting their ballots in The Gambia's unique system, where marbles are dropped into coloured metal barrels representing different candidates.
Miroslav Poche, chief EU observer and a Czech MEP, told AFP that the IEC had showed its impartiality under great pressure by standing up to Jammeh when he lost the vote last year and filed legal action against the body.
"The IEC has proved itself during the December 2016 presidential elections," he said, but added that the marble drum system was a "big challenge" for his technical support team.
It is the first time the EU has sent a fully-fledged observer mission to The Gambia, and it has bankrolled the vote at a time when state coffers are almost empty.
West African troops remain on Gambian soil three months after Jammeh's departure, and will stay until Barrow is satisfied security service reforms have removed rogue elements from its ranks.
Experts say that while the result could go feasibly in any direction, the relative novelty of a truly democratic election might distract from the unified front required to carry out the comprehensive overhaul of the state promised by Barrow.
"Given Jammeh's stranglehold over Gambian politics for the past 22 years, the country's opposition parties are inexperienced to the democratic process and relatively weak institutionally," said Joseph Siegle, director of research at the US-based Africa Center for Strategic Studies.