No cap space and a roster of 20. Think you have just proven why a very good Gardiner (if he is that in two years) is unaffordable for the Leafs.

I'd be surprised if Marleau and Martin are still on the roster in 2019-20. But it will take some sacrificial picks/prospects to move the two worst 2019 contracts.
Also I'm hopeful Gardiner can be signed for just under $6m.

I agree Martin last year Cap hit is very expensive, and it will all depend on how much it takes in the end to sign Matthews, Marner and Nylander, if cap constraints Martin will be traded after he gets his signing bonus so only 750K salary would be traded (cap hit 2.5M though so not every team can take that).
Marleau....this is a different story...it will all depend on his production as well, what if he still fit and producing like Jagr...what if not but he doesn't waive his NTC...this one is really a time will tell what is best, first let see him play in Leafs uniform.
Gardiner will be 29 when become UFA, this is one of those hard decisions management has to take, I personally wouldn't commit on a long term contract with high Cap hit (if we have that luxury wich I doubt we will) for a player who enters 30's. A 3 yr contract at 5.5M would be option I think as of now, and probably we won't even have that money, anything more is out of the question....you let him go and hope at least one of the young LHD behind can come up and produce (Dermott, Borgman, Nielssen, and/or picks Mattinen, Middleton etc..).