Place rice in bowl, fill with cool water and, using your hand, swirl rice around for a few seconds. Tip bowl and drain liquid; top with more cool water and repeat twice. Fill bowl with water again and let rice soak for 20 minutes; drain through a fine sieve.
Transfer soaked rice to small heavy pot with fitted lid and add 1⅔ cups water. Cover with lid, heat to medium-high and cook for 3 to 4 minutes, watching to avoid a boil-over. Reduce heat to low and simmer for 15 minutes. Turn off heat and let stand for 5 minutes; fluff with fork.
Heat oil in small skillet over medium heat. Add garlic and fry until fragrant, 15 seconds. In bowl, combine tuna, mayonnaise, garlic and salt; stir and crush with fork.
Divide rice among 4 large serving bowls. Top with tuna mixture, avocado, cucumber, green onions, kimchi, nori, sesame seeds and gochugaru.
Sauce: In small bowl, stir together soy sauce, sugar, mirin and 1 tbsp water. Drizzle over bowls.