Gyan is the leading scorer in the competition and the Ghana striker's 11th Champions League goal put the United Arab Emirates side in command of the last-eight tie by doubling their lead at the Hazza Bin Zayed Stadium on Tuesday.
Ismail Ahmed had opened the scoring with his first Champions League goal early in the second half and Gyan's strike left Saudi outfit Al Ittihad with it all to do in the second leg next week.
The home side made a bright start, backed by a vociferous crowd, and Gyan spurned a chance to put them in front when he headed straight at visiting goalkeeper Fawaz Al Qarni.
Zlatko Dalic's side remained on the front foot and thought they should have had a penalty when Ahmed went down under a challenge from Mohammad Al Dumeiri, but the referee saw nothing untoward.
Al Ain were in front three minutes after the break, though, when an unmarked Ahmed was on hand to volley home after being expertly picked out by Jires Kembo-Ekoko.
Gyan ought to have doubled their lead when he rounded Al Qarni and put his finish over the crossbar, but the former Sunderland man made no mistake when he put the home side in command 61 minutes in.
Omar Abdulrahman was the architect, dinking a ball over the top for Gyan and the prolific striker took a touch before firing clinically firing home.
Gyan could have made it three, but a bad touch let him down 10 minutes from time, and Al Ain withstood some late pressure to take a giant stride towards the semi-finals.
