Preheat an outdoor pellet smoker to 225°F (107°C).
Check salmon for pin bones and remove them. If desired, use a sharp knife to remove the skin from the fillet.
Place the salmon in a large dish. Drizzle with 1 tablespoon olive oil. Sprinkle 1 teaspoon garlic powder, 1 teaspoon kosher salt, and 1/2 teaspoon black pepper evenly over the salmon. Rub the seasonings in thoroughly.
Slice 2 medium lemons into thin slices. Arrange the lemon slices directly on the smoker’s grate to prevent the salmon from sticking and to add flavor. Place the salmon on top of the lemon slices.
Smoke the salmon until it reaches an internal temperature of at least 130°F (54°C). The salmon should flake easily and the juices should be translucent. This will take approximately 90 minutes, depending on the size and starting temperature of the salmon.
Remove the salmon from the smoker. Let rest for 10 minutes before serving.