The 25-year-old Ghana international will join Al-Ain after becoming unsettled on Wearside barely a year after his club record £13milion-plus switch from Rennes to the Stadium of Light.
Al-Ain club made an approach to sign Gyan on a permanent deal earlier this week, but the Black Cats were only prepared to agree a temporary move.
Manager Steve Bruce said: "Anyone who has seen Asamoah play will know that he hasn't been himself in recent months. This option suits all parties at the present time and the club is well-protected in the deal.
"Asamoah has three years remaining on his contract and of course my wish would be that he finds his spark again and we see him in red and white stripes next season in the form that first attracted us to him."
The news comes just a day after manager Steve Bruce accused "parasites" surrounding Gyan of turning his head after he was linked with a series of clubs in the wake of his goalscoring performance for Ghana against England in March.
Bruce said: "Since that game at Wembley, all the parasites, as I call them, hover around.
"People are in his ear constantly trying to engineer a deal for him.
"Certainly since the England game, when he played at Wembley so well on the night, something has been troubling him.
"It's very difficult, the constant speculation no matter what you try to quash or quell, and the people around him, the people who want to make a fast buck, shall we say, and it affects him in the end."
Gyan arrived at the Stadium of Light last summer after starring for his country at the World Cup finals in South Africa, and was a big hit on Wearside, scoring 12 goals.
However, after being linked with the likes of Tottenham, Real Madrid, Marseille and a series of big-spending Turkish clubs in recent months, he has struggled to rediscover his touch during the early weeks of the new campaign and is yet to open his account.
His departures comes with Nicklas Bendtner having joined Sunderland on loan from Arsenal for the rest of the season after clinching a deadline-day move.