Some gels can be applied like toothpaste, like Colgate’s Total Advanced Fresh Gel which is designed to remove stains for as long as it is in contact with the teeth. This kind of tooth bleaching gel, however, is often criticized for being inferior in effectiveness because it's virtually impossible to keep a significant amount of gel on your teeth after brushing (after rinsing there's not much left).
There are many kinds of tooth bleaching gel available. They vary in the strength of the bleaching agent and how they are applied. The internet and forceful advertising are the probable causes of an amazing recent growth in popularity of tooth bleaching gels even though they have been available for over ten years. And the most well known makes are made by the major toothpaste manufacturers.
Other gels are used in conjunction with a tooth bleaching tray. Some trays are generic in nature; one-size-fits-all if you will. However, their effectiveness is often questioned as not everyone's dental structure is the same. Other companies offer custom-made trays which are infinitely better than generic trays. The idea is to fill the trays with tooth bleaching gel and then fit them on one's teeth while sleeping. That way, the gels get to stay longer on one's teeth and make for a more effective whitening process.
Another form that tooth bleaching gel is applied by is tooth whitening strips. One side of the strip contains the gel and is stuck to the surface of the teeth, bleaching them. However, the strips do not get into the gaps, groves and depressions in the teeth very well. This leads to an effect called striping which is caused by these bits missing the bleaching.