First of all, Duncan Keith isn't even a replacement level NHL defenceman any longer. Secondly, your list makes no sense, and is largely comprised of the best (i.e. top 31) D-men in the league.... not "#2"s by any definition except for Team Canada or first All-Star teams.
Here is how they are #2's
Karlsson-Plays w/Burns in SJ
Ellis- Plays w/Roman Josi in NSH
Petry- Plays w/ Shea Weber in MTL
Keith- Plays w/Gustaffson CHI
Hamilton- Plays w/ that loaded D corp in Carolina
Dumba- Plays w/ Ryan Suter MIN
Ekblad- Plays with Yandle- FLO
Dunn- Plays on the towers of death that is STL defense corps
Brodie- Giordano in CGY
Myers- Plays w/ Big Buff in WPG
These are all #2' for their teams and they are all better than Gardiner...that isn't to say Gardiner (when used properly) isn't a very good defense-man because he is...but his mental makeup isn't a fit for the microscope that is playing for the Toronto Maple Leafs...besides how are we going to use him? Rielly and Barrie are clear cut 1 and 2 I think Muzzin comes in ahead of him as well...which sticks him into a #4-7 Dman we have enough puck movers at this point...
Rielly- Ceci
Muzzin- Barrie
Harpur-Dermott (some plug until Dermott is better) we can't afford 5M on a 3rd line defender who won't play PP (Rielly,Barrie and Muzzin all come before him) doesn't PK and makes major mistakes at terrible times...let's move on from Gardiner...he's been here and loyal but sometimes it's better to move on and start somewhere new...I hear the Isles play in a pretty empty building or Arizona