Pat the salmon fillet dry with paper towels. Brush it all over with soy sauce. In a small bowl, combine the garlic salt, lemon pepper, and poultry seasoning. Rub the seasoning mixture evenly over the fish. Let it rest at room temperature while you prepare the grill and glaze.
Set up a charcoal or gas grill for indirect heat and preheat to 250–275°F (120–135°C). Adjust vents or burners to maintain a steady, low temperature.
In a small saucepan over medium heat, melt the butter. Add the minced garlic and sauté for 1 to 2 minutes, until fragrant. Stir in the brown sugar, orange zest, and orange juice. Add the soy sauce and a pinch of each seasoning used on the fish. Bring to a boil, then reduce the heat and simmer for 5 to 7 minutes, stirring often, until slightly thickened.
Place the salmon skin-side down on the cooler side of the grill. Close the lid and smoke for 20 to 30 minutes, depending on the thickness, until the internal temperature reaches 135°F (57°C).
Brush the orange glaze generously over the fish. Close the lid and cook for another 10 minutes to let the glaze set and caramelize.
Remove the salmon from the grill with a spatula or fish turner and let it rest for 5 minutes. Slice and serve warm with extra glaze, if desired.