Fowler is better than Gardiner IMO, but I base that on my feeling that Fowler would perform admirably in a role similar to what Gardiner has for the Leafs. A statistical analysis favors Gardiner quite clearly, so that is just my gut feeling. I don't see Fowler being far ahead though.
As for Gardiner as a PK player. It's actually quite funny because the times I've seen him on the PK, he's been so unorthodox that he surprised opponents. He'd use his supreme mobility to launch quick strikes on puck receivers on the half-boards. It was amusing to see. I don't think it was very translatable to a regular PK role though.
A PK D-man uses a lot less mobility and, most of all, transition ability compared to an ES D-man. More focus is put on the ability to handle forwards in front of the net, and more on positioning and stick work over cover ability.
The only part of Gardiner's game that would translate well to the PK would be his above average stick work. But that's fine. He's still a guy who can fill a 20 minute role and excel in it.