A driver identified as Bai L., in line with German privacy rules, was a member of Jammeh's elite guard known as "the junglers" between 2003 and 2006.
In 2022, German prosecutors said they filed charges against Bai L, who on three occasions drove officers to locations where they fired at opponents of Jammeh.
Jammeh has been accused of mass killings, dumping bodies in wells, falsely claiming to have a herbal cure for AIDS, and for rape. He is yet to publicly comment on the allegations and is living in exile in Equatorial Guinea.
A spokesperson for the German court said Bai L. denied any involvement and that the verdict can still be appealed
Germany recognizes universal jurisdiction for serious crimes, allowing the accused to be tried there even though the offenses happened in Gambia.
The German court convicted Bai L. for his role in the murders of at least three of Jammeh's opponents, including journalist Deyda Hydara shot dead in 2004 on the outskirts of the capital Banjul, and an attack on a lawyer in 2003.
His son Baba Hydara welcomed the verdict as a milestone paving the way for "many more people to be held accountable."
Reporters Without Borders said it was the first criminal trial relating to crimes against humanity under Jammeh's rule.
"This trial is an important sign for the people of Gambia," Nicola Bier, a legal adviser for the non-governmental organization, said.
Gambia's government said last year that it would seek to prosecute Jammeh for killings and other suspected crimes from, following a recommendation from a truth and reconciliation commission.
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