Douglas was locked in a head-to-head battle with Russia's Viktoria Komova but she edged her opponent into silver medal position with a total of 62.232, with Aliya Mustafina claiming bronze.
Douglas rocketed into an early lead on Vault thanks to her 15.966-scoring Amanar.
She showed why she was nicknamed 'The Flying Squirrel' on the Uneven Bars as she posted a strong score, beaten only by her two Russian opponents, Komova and Mustafina.
Komova, the reigning world Uneven Bars champion, was in the same group as Douglas, the USA's Aly Raisman and Mustafina. She made sure she kept it tight at the top with a strong Vault of her own and 15.966 on the Bars - just below the score of her team-mate's 16.100.
Mustafina then saw her hopes of gold crumble after she fell off the Uneven Bars as Douglas shone, retaking the lead from Komova with only the Floor remaining.
He told me my training will not go in vain. So there I was, just cruising and now I am on cloud nine.Gabrielle Douglas
Douglas produced a fantastic Floor performance to claim her second gold of the Games - after helping USA win the Team final - with a crowd-pleasing 15.033 routine.
It was enough to claim the title ahead of Komova, the last gymnast to compete.
Douglas said: 'I just wanted to go out there and show everyone what I was capable of.
'My coach has prepared me well. He told me my training will not go in vain. So there I was, just cruising and now I am on cloud nine.'