Gyan has struggled to rediscover his best form this season after a successful first year on Wearside, and Bruce believes speculation linking the 25-year-old to a transfer is to blame.
Bruce believes Gyan has become distracted by the possibility of a move following his oustanding display for Ghana in a friendly against England at Wembley in March.
"Since that game at Wembley, all the parasites, as I call them, hover around," Bruce told a press conference.
"People are in his ear constantly trying to engineer a deal for 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.
"He was going from Real Madrid to Bayern Munich to Valencia to Atletico Madrid.
"We have had a discussion with him and he needs now to focus and get himself back on track again and be the Asamoah Gyan we know he is capable of.
"These things affect these players, and it has affected Asa.
"I had a conversation with him two days ago to say, 'Look, the window has now closed, Asa. Right, we need to see you back playing again and back the way you know you can play.'"
Gyan has yet to score this season and has been ruled out of Saturday's visit of Chelsea after suffering a hamstring injury.