Mourners lined the streets of Banjul as police passed by carrying the flag-draped coffin of Solo Sandeng, in a ceremony that revived memories of Jammeh's brutal rule.
Sandeng died in custody in April 2016 at the age of 57 after organizing a protest calling for the return of democracy.
His death was a catalyst for uniting the country's fractured opposition and driving a wave of pro-democracy protests which ultimately led to Jammeh's fall.
Justice Minister Dawda Jallow paid tribute to Sandeng, describing him as "a man who paid the ultimate price in fighting for the cause he genuinely believed in.
"He has left an indelible mark in the political history of this country.
"His legacy will continue to live on in the history of our democratic transition as a nation."
Sandeng's body was exhumed in March 2017. Six members of Jammeh's National Intelligence Agency (NIA) were convicted of his murder in July 2022, with the agency's former director being sentenced to death.
The Gambian authorities have charged Jammeh with murder, rape, torture and corruption.
However, he retains substantial clout back home.
Barrow's government, which won elections in 2016 and again in 2021, has yet to fully implement recommendations made by a truth and reconciliation commission in in 2021.