Combine 2 cups water with the quinoa, garlic and a pinch each of salt and pepper in a medium saucepan over medium-high heat. Bring to a boil, then reduce the heat, cover and simmer until the quinoa is tender and the water is absorbed, about 15 minutes. Fluff with a fork. Let cool slightly, about 10 minutes.
Meanwhile, stir the sugar and vinegar in a medium bowl until dissolved. Whisk in the olive oil and season with salt and pepper. Transfer half the dressing to another medium bowl. Add the zucchini, both peas and the light green parts of the scallions to one bowl; toss. Add the shrimp to the other bowl and toss.
Divide the quinoa among bowls and top with the vegetable mixture and shrimp. Sprinkle with the scallion greens and basil.