Gardner made the switch last week.
McLeish said: "Gary is a player that everybody told me about when I came here and how he could burst into the first team almost straight away.
"But people forget he was out with a cruciate ligament injury for about a year. He is now starting to show the potential he has got.
"We are now seeing that in his performances and also getting a chance for his country at under-21 level.
"He is also chipping in with goals which is fantastic for his confidence."