The Wales international has impressed Redknapp since returning to the side in January.
“Gareth is playing superbly, he’s been in great form,” Harry told the club’s official website.
“I didn’t want to shift him from the left-back position because he’s been outstanding there but we didn’t have many spare bodies at Fulham.
“He can play wide left or left-back anyway, he can do both jobs and he was a real threat. He’s in great form and is full of confidence.”
“He’s got bigger and stronger and people forget he’s still only a young lad,” he continued.
“He’s toughened up a lot, his attitude is stronger, he’s physically stronger, mentally stronger and he has terrific ability, pace, a great left foot – he’s got everything.
“I think he will go on to be one of the best left-backs or left-sided players in the modern game.”