If using frozen salmon, thaw in the refrigerator overnight and drain.
In a mixing bowl combine soy sauce, rice vinegar, sesame oil, sweetener, garlic powder, Korean red pepper powder (gochugaru), and sliced green onion; mix well.
Divide the mixture in half: one half for marinating the fish and the other half reserved as the dipping sauce.
Pat the salmon fillets dry and place them into a Ziplock bag or airtight container; pour the marinade half over the fish, seal, and refrigerate for about 1 hour.
When ready to cook, remove the salmon from the fridge, brush the top (skinless side if applicable) generously with olive oil, and let the fish come closer to room temperature while you preheat the grill.
Preheat the grill (or grill pan) to medium-high heat and grease the grates with oil or non-stick spray to prevent sticking.
Place each fillet on the grill with the top side down at an angle to the grates; grill 2–3 minutes until grill marks appear.
Carefully lift and rotate each fillet to the opposite angle to create crosshatch marks and grill another 2–3 minutes.
Brush the other side of the salmon with more olive oil, flip the fillets over, and grill about 3–4 minutes more, or until the fish is firm and reaches about 130°F (55°C) for medium; total grill time will be about 8–10 minutes depending on thickness.
Remove salmon from the grill, let rest briefly, and serve hot with the reserved dipping sauce on the side.
