Make the sauce: Whisk the gochujang, soy sauce, sesame oil, lemon juice, sugar, ginger and garlic in a small bowl.
Prepare the salmon: Remove 3 tablespoons of the sauce to a large bowl and add the salmon; rub all over to coat. Cover and refrigerate 30 minutes. Set aside the remaining sauce for basting.
Preheat a grill to medium high. Oil the grates. Brush the cabbage wedges all over with 2 tablespoons vegetable oil and season with salt and pepper. Remove the salmon from the marinade; pat dry with a paper towel. Season with salt and pepper and brush both sides with the remaining 1 tablespoon vegetable oil.
Grill the salmon, skin-side up, until well marked, 4 to 5 minutes. Flip and brush with the reserved sauce. Grill until cooked through, 3 to 5 minutes. Meanwhile, grill the cabbage wedges, flipping often, until crisp-tender, about 10 minutes. Transfer the salmon and cabbage to plates; sprinkle with scallions and drizzle with sesame oil. Serve with rice.