The pressures on the inside and outside of the inflated balloon are nearly equal. The pressure on the outside is the constant atmospheric pressure. The Law of Gay-Lussac therefore applies. The pressure in the freezer is atmospheric pressure, the temperature in the freezer is lower that the outside temperature, so the volume of the balloon decreases when it is placed into the freezer. The earth atmosphere is a gas in the presence of gravity.
Gravity prevents the atmosphere from escaping.
The atmosphere is a fluid, and the pressure in this fluid is a function of the altitude. But the atmosphere is a compressible gas. Let us consider a column of gas containing r particle particles of mass m per unit volume near the Earth's surface. The pressure decreases exponentially with altitude. The pressure, density, and temperature of the atmosphere decrease as the altitude increases.