Preheat an oven to 350 degrees F (175 degrees C). Place the squash halves into a large baking dish with the cut-sides facing down.
Bake in the preheated oven until you can easily cut into the skin side with a knife, about 30 minutes; remove from oven and set aside to cool.
Toss the cooled spaghetti squash, the tomatoes, olives, cucumbers, red onion, and garlic together in a large bowl until evenly mixed.
Stir the lemon juice and lemon zest together in a small bowl; slowly pour the olive oil into the lemon juice mixture while whisking vigorously. Season with garlic salt and pepper; drizzle over the spaghetti squash mixture and toss to coat. Refrigerate at least 2 hours before serving.
