Gallen brought Sterling through at QPR before he was sold to the Reds.
"We always knew he was good," Gallen told the Daily Mail. "He had many nicknames when he played for us. One of them was simply "The Magician'. Another was 'Raheem Park Rangers' because sometimes he would carry the team on his own.
"I moved him up to the Under 16s when he was 14 and he was the best player on the pitch. He came on as a sub against Millwall and there was this long ball from the goalkeeper. He watched it drop over his shoulder and volleyed it straight into the top corner. I was glad no scouts were there that day.
"This became normal. His ability is frightening. Balance, speed, awareness. No-one I've ever seen compares to him. He's a fantastic player and I really hope he does well."