Gardiner is good. But from his draft alone, Doughty, Pietrangelo, Karlsson, Myers, MDZ, And Carlson are all definitely better than Gardiner at the present time. Schultz and Bogosian probably are, and then Voynov, Hamonic, etc aree pretty similar.
Look at all D aged 22 or younger, and consider where Gardiner realistically ranks among them. Then consider all other top D aged 23-30 who will also be around for a long time and include them in that ranking.
Considering this trade proposal even includes Dumba coming back in it, + Granlund (widely considered a top 5 or so prospect in the world), Coyle (better prospect than Kadri) + a 1st, i dont think you should get held up by Gardiner. Gardiner is a nice young player to havde, but at the end of the day he gets overrated around here quite a bit.