Jays are going to win the DS in 3 games anyways, so Gausman and Bieber will have plenty of rest to start Game #1 and #2 of the ALCS.
On a more serious note, you have to take the DS one game at a time. If Gausman is your Game #1 starter, I think you have to roll with Bieber in Game #2. What happens if the Jays lose Game #1 let's say? That Game #2 win is imperative and you need your best arm pitching. Can't save Bieber for a Game #3 in that scenario. Plus, getting a 2-0 lead would be huge if possible.