Up to you if you want to make it to Fogo or not, I would argue that you'll have as good or better experience visiting Twillingate without having to deal with the uncertainty with the Fogo Island Ferry. Don't plan to spend much time in Gander, really just a hub for area and not much to do.
As others have said, you could go to the west coast with that amount of time, but it is a lot of driving. I would probably focus on the east coast, maybe go down around the Irish Loop during your St. John's time or visit Trinity/Bonavista as part of the trek to St. Pierre. In and around St. John's, you can go whale/puffin watching in Bay Bulls, hike around Signal Hill/Battery, visit Cape Spear, the Rooms, etc. Plenty to do to fill the time.
Be sure to be prepared for nice and not so nice weather. Summer in St. John's can be beautiful and it can be cold, wet and very windy.
If you have other questions, would be happy to help. Originally from YYT and have spent a lot of time in YQX.