I've never understood the point of trading Gardiner, would leave a massive hole we can't fill. And he is on an excellent contract. It would need an enormous return. Don't see it.
And Kadri I believe has 2 years left on his RFA, which means we should be able to do a very fair 4 or 5 year deal. Again, if we play this smart in a couple of years his contract will also look very good.
I would focus now on a free agent this summer (mm, which one?) a good draft, and developing our talent. Picking up a KHL defenseman also might help.
If you only see him as a #3 then everything depends on the return. In a sellers marketing and especially one that prioritizes good D you'd get more for him than you otherwise would. That increases the chances of 'winning' the trade, and because we're in pure rebuild mode right now a trade we 'win' does more for the rebuild long term. Ditto with Kadri - same idea. To hell with the short-term hole. That's the idea, anyway.
Edmonton, our greatest fear in terms of rebuild states, drafted poorly. in '09 they took MPS when De Haan, Ellis, and Kulikov went right after. In '10 they drafted Pitlick 31st OA when guys like Justin Faulk and Alex Petrovic went right after. Yes, they ended up also drafting another D later in the 2nd round - Martin Marincin.
In 2011 they drafted RNH and Klefbom, arguably a good draft for them. Again their 31st OA pick was taken to Davil Musil, who's done nothing, while right below him there's a TON of talent that went right after: Rattie, Grimaldi, Scott Mayfield (D), Tomas Jurco, Clendening (D), Boone Jenner, John Gibson, Khoklhochev, Victor Rask, Brandon Saad, and Markus Granlund all went from 32 to 45. Talk about a stroke of (bad) luck drafting Musil. 2012, they draft Yakupov, probably the silliest thing to do given their need for talent on the blueline at the time. Ryan Murray goes 2nd, Rielly 5th, Hampus Lindholm 6th, Dumba 7th, Pouliot 8th, Trouba 9th... just wow. Then, in keeping with tradition on horrible early-second-round picks they draft LW Mitch Moroz 32nd overall. Granted, not a lot of good talent in that 2nd round that year, so we'll give them that at least. At 2013 and beyond we're a bit too close to say how their 1st rounder Nurse will turn out. Looks to be promising though...
But my point is, Edmonton's draft history is questionable at best over the last 8 years. THATS why they're still where they are - not because they went through a rebuild and made holes in the lineup (like we'd be doing trading Gardiner).