Use a small paring knife to puncture the spaghetti squash a few times all over. Place on a baking sheet and bake until tender, about 1 hour. Let cool until cool enough to handle, then use a large knife to slice the squash open lengthwise. Scrape out the seeds with a fork and discard. Continue to use the fork to pull the flesh from its peel. (Leave the oven on.)
Wrap the squash in a clean dishtowel and squeeze out all the excess water. (There will be quite a bit of water in the squash. It is important to squeeze out as much as you can so that the crust will have a better texture.)
Whisk the eggs together with the shredded mozzarella and garlic salt in a large mixing bowl. Stir in the spaghetti squash until well combined.
Line a baking sheet with parchment paper and spray with nonstick cooking spray. Transfer the squash to the center of the baking sheet and spread into a 10-inch circle. Bake until golden brown, about 20 minutes.
Spread the pizza sauce on the crust and top with the desired toppings. Bake until the cheese is melted and bubbly, an additional 10 minutes.