Dettol contains phenols, which can be toxic to animals if they are ingested or inhaled. Phenols can cause a range of health problems, including skin irritation, eye damage, respiratory problems, and even death.
In addition, Dettol is not effective at killing all of the bacteria that can live in a gerbil cage. Some bacteria are resistant to Dettol, and can actually multiply and grow in its presence. This can make it more difficult to keep your gerbils healthy.