138 doesn't go to anywhere in Newfoundland. You'll need to be more specific on your routing.
Two usual ways to get to Newfoundland and Labrador by bike or car.
1) Ferry from North Sydney:
Most people take the Marine Atlantic Ferry from North Sydney Nova Scotia to Port aux Basques in southwestern Newfoundland. You can also take a ferry from North Sydney to Argentia on the southeastern side of the island. Port aux Basques is a shorter run (and significantly cheaper) and goes twice per day during the summer (one day has three sailings). The ferry to Argentia goes three times per week. For either route, make sure you make reservations well in advance - especially if you want a cabin.
https://www.marineatlantic.ca/
2) Via the TransLab Highway
Various ways to get to the start of the route, but from Baie Comeau, you take Quebec Route 379 North to the Quebec-Newfoundland/Labrador border near Fermont. Labrador City is just on the Newfoundland/Labrador side. Note: Two separate land masses but it is one Province called Newfoundland and Labrador - abbreviation is NL. Labrador is the mainland part and Newfoundland is the island.
From Lab City, you take Hwy 500 to Happy Valley Goose Bay and then Hwy 510 to Blanc Sablon (actually just over the border to Quebec on the east coast of Labrador. You can take a relatively short ferry from Blanc Sablon to St. Barbe near the top of the Northern Peninsula. You need to book this one in advance as well although not as far in advance.
The TransLab highway is paved (1125 km) all the way from Labrador City to Blanc Sablon.
Quebec 389 is mostly paved but there is around 170 km (out of 587 km) that is unpaved. Most of it isn't terrible but it will depend on weather and recent grading. I did this route last year on a BMW F700GS. Would have been doable on an FJR but there might be a few pucker moments. TransLab highway is quite remote and mostly no cell service except in towns. You will want to make sure you have accommodations worked out in advance.
An alternate route is, I suppose, taking the 138 along the north shore of the St. Lawrence River through Baie Comeau through to the end of the line at Kegaska. (Mostly paved, some dirt.) Last I heard it was in decent condition) There is a coastal ferry that eventually works its way around to Blanc Sablon where you can catch a boat to Newfoundland. I know almost nothing about this ferry route so you will have to do some research on sailings, reservations, costs etc. This is NOT the most common way to get there.
Start here for coastal ferry information:
https://www.rome2rio.com/s/Kegaska-QC/Blanc-Sablon#:~:text=Desgagnes operates a car ferry,and the journey takes 33h.
I have done more than a half dozen trips to Newfoundland by motorcycle - one of my favorite motorcycle destinations. I usually take 10 days or so on the island and have visited almost every corner on paved routes. Last year's trip was supposed to include some roads (dirt) off the beaten path but it got cut short due to an electrical problem with my bike and a poor long-range forecast. I mostly camp at provincial or private campgrounds. I may be going back next year (mid-to-late July). Lots of stuff to see. Keep in mind that this is a large island stuck out in the Atlantic ocean and weather can be an issue (or it might be fantastic). Driving across the island from the ferry at Port aux Basques to the provincial capital of St. John's is 900 kilometers (9.5 hours plus stops). Most places to see require a trip out-and-back from the main highway.
Plan it out a bit and let me know how the trip is shaping up and I can give you an idea of what you really should see. Let me know duration (on the island), camping vs hotelling, interests (scenery, culture, food, history) and what sorts of roads you want to ride. You won't find Newfoundland a twisty road paradise but it is an interesting and unique destination.
You can order a Newfoundland Travellers Guide which has a map and tons of information. (You can view all of the information on-line as well.)
https://www.newfoundlandlabrador.com/trip-ideas/travellers-guide
Again, depending on your schedule and time available, there are some places worth seeing going to (or returning from) Newfoundland.
Hope this helps!
Oh, a point of confusion to some people visiting Atlantic Canada. St. John's is the capital of Newfoundland and Labrador and is located on the eastern end of the island of Newfoundland. Saint John is a city on the Bay of Fundy in southwestern New Brunswick... (St. is abbreviated and apostrophe "s" for the one in Newfoundland.)