He does pretty well against top level competition considering how often he has to do it and who he's doing it with. It's also not like his offensive game is being promoted at all. I think people look at the stats and make judgements about him but do not realize how integral the guy is for this defense and how difficult it would be to find someone like him at his price.
Like I said, neither should be looked at being dealt right now. Besides that main core group of Matthews, Marner, Rielly, Andersen and Tavares, they are with Nylander as guys who are next to certainly not going to be dealt right now. It would take a lot (either really poor cap management or an amazing offer) for the Leafs to consider dealing either any time, because neither are going to be easy to replace.
Until he leave for UFA, Gardiner should be the first high end value guy on the trade block for the Leafs. His offense is not super easy to replace, but given his gaps in other areas and that the Leafs have offensively capable defensemen like Rielly, Dermott and Zaitsev (plus possibly the other guy they bring in if they do deal Gardiner), it would not be difficult to deal with.