Heat the oven to 425°F (220°C). Line a baking sheet pan with aluminum foil. If using wooden skewers, soak them in water.
In a medium bowl, combine the salmon, zucchini, red pepper, 2 tablespoons of the soy sauce, and 1 tablespoon of the sesame oil. Toss gently to coat. Cover and refrigerate.
In a heavy-bottomed medium pot, combine the remaining 1/2 cup soy sauce and 1 tablespoon sesame oil plus 1 cup water, the sweetener, onion powder, and ginger. Bring to a boil over medium-high heat, then reduce to a low boil and cook, stirring occasionally, until the sauce is reduced by half and slightly syrupy, 8 to 10 minutes. Sprinkle in the xanthan gum, whisk to combine and simmer over moderate heat until the sauce has thickened, about 1 minute more. Remove from heat, stir in the sesame seeds. Set aside.
Remove the wooden skewers from the water, then assemble the skewers: thread the marinated salmon, zucchini, and red pepper evenly among the 4 skewers, then arrange the skewers in one layer on the prepared baking sheet. Roast for 4 minutes, turn over the skewers and brush or drizzle with half of the sauce. Roast for 4 minutes more, turn the skewers again and brush with the remaining sauce. Return to the oven until the salmon is just cooked through and gently flakes, 2 to 3 more minutes more. Serve right away.