Preheat an oven to 375 degrees and spray a medium sized (oven safe) casserole dish with non-stick cooking spray.
In a large saucepan preheated to medium heat, melt the butter. Next, add in the garlic and cook until fragrant; about 1 minute. Whisk in the flour and cook for another minute until a roux forms.
Next, pour in the milk gradually and begin to stir. Bring the mixture to a simmer and stir until thickened about 2 minutes.
Turn off the heat and stir in the 1/2 cup gruyere, 1/2 cup mozzarella, and 1/4 cup parmesan. Keep stirring until the all the cheese has melted. Season with salt and pepper.
Add a layer of zucchini rounds to the bottom of the casserole dish. Pour about 1/3 of cheese mixture over the zucchini. Repeat process until all zucchini have been used.
Once the layers have been formed top the zucchini with remaining cheeses and bake for about 20 to 25 minutes or until golden brown. Garnish with fresh parsley. Serve and enjoy!