The court on Tuesday found Ike Ngouoni guilty of both charges between 2017 and 2019, when he was director of communications for President Ali Bongo Ondimba, they said.
It also slapped him with a fine of 50 million CFA francs (around $77,000), as well as a further 5 billion CFA francs ($7.7 million) in damages, they said.
His lawyer Carole Moussavou described the verdict as "unfair", claiming the funds her client was managing were "not public as they did not come from the treasury but from a private account".
A second defense lawyer, Anges-Kevin Nzigou, said he would appeal the verdict in the coming days.
"It is not a judicial affair, but a political affair," he said.
The prosecution had requested up to 15 years in jail on Monday.
Ngouoni was arrested in 2019 as part of a wider anti-graft crackdown against corruption in the oil-rich central African state.
Brice Laccruche Alihanga, a once-powerful chief of staff under Ondimba, was also detained the same year.
The French-Gabonese ex-cabinet director was sentenced to five years in prison last year over "improperly issuing administrative documents."
Patrichi Tanasa, the former head of the public Gabon Oil Company, was also arrested in 2019.
Last week, he was sentenced to 12 years in jail for "embezzlement of public funds".
He was ordered to pay a fine of 70 million CFA francs ($107,000) and reimburse the Gabonese state 20 billion CFA francs ($30.8 million) in damages, the prosecution said.