Cook squash and 1/2 teaspoon salt in boiling water to cover in a 2-quart saucepan 10 minutes. Drain and cool.
Cut squash in half lengthwise. Scoop out pulp, and reserve, leaving 1/4-inch-thick shells.
Melt butter in saucepan over medium heat; add carrot, bell pepper, and onion, and sauté until crisp-tender. Add garlic salt, pepper, and soy sauce; cook, stirring often, until vegetables are tender. Stir in reserved pulp. Spoon into shells, and place in a 13- x 9-inch pan.
Bake at 325° for 10 to 15 minutes.