Brush both sides of squash slices with 3 tablespoons of the oil; sprinkle with salt and pepper.
For a charcoal grill, grill squash, cut sides up, on the rack of an uncovered grill directly over medium coals about 6 minutes or until crisp-tender, turning once. (For a gas grill, preheat grill. Reduce heat to medium. Place squash on grill rack over heat. Cover and grill as directed.)
On a serving platter arrange squash, tomatoes, and basil. Drizzle the remaining 2 tablespoons oil over vegetables just before serving.