The pressure of the gas would increase.
Explanation:
Using the ideal gas formulae we can evaluate this fact;
P₁V₁/T₁ = P₂V₂/T₂
We can assume that the volume is reduced by half (the T₁ and T₂ will cancel out because they remain constant);
P₁V₁ = P₂ * 0.5V₁
P₁V₁/0.5V₁ = P₂
P₁/0.5 = P₂
You can observe that the pressure will also increase – diving a number by a fraction or decimal number increases the resultant value. In this case, the pressure will increase by 2 fold