Teeth whitening gel is good for up to 2 years and in 2 years the gel expires and will not work properly to clean and whiten your teeth.
So be sure to use the teeth whitening gel within 2 years for it to be effective and if you haven't used the teeth whitening within 2 years then the gel should be thrown out and not used.
It's not gonna hurt you to use the expired teeth whitening gel but it won't give good or prominent results.