Preheat a grill to medium high. Lightly brush the grates with vegetable oil. Toss the tomatoes, cucumber, onion, ginger, 1/2 teaspoon salt and a few grinds of pepper in a medium bowl.
Rub the top and sides of the salmon with the garam masala; season generously with salt and pepper. Brush both sides of the bread and the cut sides of the lemons with the vegetable oil. Grill the salmon until charred but still slightly pink in the center, 4 to 5 minutes per side. Grill the lemons until juicy and well charred, about 2 minutes per side. Grill the bread until toasted, about 1 minute per side.
Squeeze 2 grilled lemon halves into the tomato-cucumber salad, add the avocado and toss. Divide among plates. Add a piece of salmon and a grilled lemon half to each plate. Cut the bread into wedges and serve with the salmon.