Rebuilding if they both leave

The Flames won't rebuild. Ownership simply won't allow it. They just collected a nice cheque from 7 home playoff games. That will be the mandate regardless of who they have, rightly or wrongly. The only time the Flames rebuild is when they have no other choice.

Even if Gaudreau leaves, this summer and Tkachuk leaves the followings summer, the Flames have no reason "to rebuild". Just spend that the cap you were going to spend on those two guys on other players.
It's the job of GMs to find. value since most teams spend to the cap or close to it anyway.

In this scenario though, they have to trade Mangiapane and Lindholm as well.