Preheat a grill or grill pan to high. Drizzle both sides of bread with 3 tablespoons oil and season with salt and pepper. Grill on both sides until nicely marked and somewhat crispy, about 5 minutes. Remove from grill, set aside to cool.
In a large bowl, toss together tomatoes, peppers, cucumber, garlic, capers, parsley and basil. Drizzle in the vinegars, remaining 1 tablespoon of oil and season with salt and pepper. Toss again. Let sit for 15 to 20 minutes in the refrigerator. When ready to serve, cut bread into 1-inch cubes and toss gently with the vegetables. Gently fold in goat cheese and avocado and adjust seasoning, if necessary.