It's not bad or harmful to use expired teeth whitener gel but when you do use the expired teeth whitener gel it won't be very effective.
When the teeth whitener expires and gets older the hydrogen peroxide in the teeth whitener breaks down and leaves only water and oxygen, which won't brighten your teeth very good or at all.
So while not harmful to use expired teeth whitener gel it's not a good idea to use the expired teeth whitener.
If you want to get whiter teeth you need to and should use fresh non expired teeth whitener gel.
Teeth whitener gel that has just recently expired as in expired a month or two ago is usually still pretty effective.
But if the teeth whitener gel is expired longer than a few months to a year or more then it's not gonna be very effective or effective at all for whitening teeth.
For best results it's best to buy some new teeth whitener gel or teeth whitening products and use them before they expire.