Yes you can brush your teeth immediately after teeth whitening and it won't cause any harm to your teeth or mess with the teeth whitening.
Teeth whitening gel when used properly will not damage your gums.
However it's normal to have some irritation of the gums from teeth whitening gel though but there should be no damage to your gums when the teeth whitening gel is used properly.
When using teeth whitening gel you should use the teeth whitening trays for best results.
However I have applied some teeth whitening gel to my teeth directly and it helped whiten my teeth and I brushed it off a few hours later.
You can use expired teeth whitening gel and it's safe to do so.
But expired teeth whitening gel does not work as good as fresh non expired teeth whitening gel does.
So using expired teeth whitening gel means you won't get your teeth as white as you would if you use fresh non expired teeth whitening gel.
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.