People on this forum have a weird idea of what makes a top pairing or #1 defenseman. It would seem we need a player who:
-never makes positional errors
-never gets beaten 1-on-1
-never loses a board battle
-throws bone crunching hits
-is never the victim of an unlucky bounce
Look around the league at guys who are not only top pairing d-men, not only #1s, but considered franchise defensemen. Let's say Karlsson, Doughty, Subban, Weber, Chara 3+ years ago, Suter. They get beaten sometimes. They screw up routine plays once in a while. They gamble for offense, sometimes when they shouldn't. They play the most minutes against the best players, of course they get bested sometimes.
These guys would be shredded on this forum if they played for the Leafs. Chara's performane the year Boston won the Cup would have made him trade-bait.
There are no flawless defensemen. So how much more do you need Gardiner to do before you think he's worth his mistakes? The guy has shown he's capable of 26+ minutes/game against top competition. He's shown he can drive offense, from his own zone or the opponents' blueline.
He reminds me more and more of Kaberle, if Kaberle had been more willing to use his wicked slapper. Not overly physical, but solid positionally and with great stick-work. Takes few penalties. Is such a smooth skater that some people think he's not putting in enough effort. High-end breakout playmaker, excels under pressure and in big games. He even has Kaberle's great ability to look off his pass targets, though Gardiner could learn a thing or two from Kaberle about head/body fakes.