Answer is: final temperature of the gas is 60 K.
Gay-Lussac's Law states that the pressure of a given amount of gas held at constant volume is directly proportional to the Kelvin temperature.
p₁/T₁ = p₂/T₂.
p₁ = 500kPa.
T₁ = 300 K.
p₂ = 100 kPa.
500 kPa/300K = 100 kPa/T₂.
T₂ = 300K · 100kPa ÷ 500 kPa.
T₂ = 60 K.