Former military coup leader Jammeh scored a landslide 72 percent victory to extend his 17 year-rule over the tiny West African country, criticized for alleged human rights abuses and press-muzzling.
"Although provision was made for equal access of all political parties and candidates to the public media, the actual coverage was strongly weighted in favor of the candidate of the ruling party," the AU observer mission concluded.
"The gross imbalance in the financial and material capability of the candidates may have resulted in the lack of adequate visibility of the United Democratic Party (UDP) and the Independent candidates," it said of his main challengers.
However the continental grouping found that there were no acts of intimidation during voting on Friday and concluded that despite the failings, "the results are a true reflection of the will of the sovereign people of The Gambia."
Results showed Jammeh won 470,550 votes, while his closest rival Ousainou Darboe got 114,177 votes, or 17 percent, Carayol said. Independent candidate Amath Bah scored 11 percent. Many analysts saw the incumbent's victory as a foregone conclusion.
Darboe has urged Gambians to reject the election as rigged, while Bah complained of insufficient access to media and funds to campaign properly.
One of Africa's most controversial rulers, Jammeh announced in 2007 that he had a herbal concoction that cured AIDS, but only on Thursdays, a claim derided by health experts. He has been criticized for reported threats to human rights groups and a 2008 order for all homosexuals to leave Gambia.
Jammeh's standing abroad has been further strained by spats with Senegal and Guinea, while the West African body ECOWAS said this week it would not send an observer mission to the polls because it doubted they would be free and fair.
Gambia is one of only handful of African states not to have diplomatic ties with China because of its recognition of Taiwan.
(Reporting by Pap Saine; Writing by Mark John; Editing by Andrew Roche)