Not even in the same ballpark simply because they see the game differently.
Before I explain there strength and weaknessess, let me say this that, Gardiner has actually looked better as of late(last 5 or more games).
Gardiner skating is figure-skater like, he can literally glide over the ice from coast to coast and toy with opposing forwards and defenders however because he has low IQ, he often refuses to shoot or create odd-man rushes the other way because instead of just finishing the play, he will try to keep the possession of the puck until he no longer can, which results in leaving one defender back quite often, also even his game has been at best, he has always been bad at decision making(makes bad pinches at times and even makes bad passess which leads to odd man rushes). hence, his
hockey sense is average to below average, often fumbles the puck, literally if he had a hockey sense of Rielly, he would at worst be regarded as a good top 4 d-man.
While
Rielly on the hand is an explosive skater who sees the ice tremendously, his ability to move the puck is outstanding, and he is got
great hockey sense. Not to mention, Rielly played very well last year and hasn't played anything close to bad this year as well. Even with Carlyle, he was playing average, but certainly not anywhere where near bad. I would say he was playing a safe and sound game as opposed to high-tempo.
I have seen every single Leaf game that both these guys have dressed in. You won't find a single Leafs fans who has criticized Rielly the same way they did with Gardiner since he played his first game as a Leaf.
Some of us may have been dissapointed with Rielly in the first half of this year because he frequently left more to be desired, he wasn't displaying the high offensive creativity that he was drafted for and didn't care to jump in plays and make nice end to rushes that he did last year until recently. Gardiner on the other hand had been quite of a disappointment because he makes too many dumb mistakes at both ends of the ice over and over again but lately he is looked ok.