It makes sense in practice value-wise but I really can't see us subtracting from our core to face Boston or Tampa in the playoffs.
Gardiner is not a part of the core. If he was, we are even less likely to beat either of those teams. He's not a core player, and is not valued like one either.
Matthews, Tavares, Marner, Rielly, Andersen is your core.
Kadri is kind of on his own island, although you can argue putting Kapanen and Dermott in there now as well. Unlikely to get moved without a significant overpayment coming the Leafs' way.
Zaitsev, Nylander, Gardiner is your next tier. Good players, but easily expendable in the right deal.
Then Hyman/Marleau/Johnsson. Same as above, but not as valuable or expensive on the cap.
etc.
So Gardiner is pretty much just squeaking in the top 10 of most important players on this team, if not outside of the top 10 altogether. I'd say he's pretty expendable.