Golden Teacher (short form GT Mushroom) is a popular cultivated strain of Psilocybe cubensis, one of the world’s best-known hallucinogenic mushrooms. The name reflects the light brown, yellowish, gold top caps of this strain (wild P. cubensis is usually a little darker) and also the insight many users report receiving.
Whether Golden Teacher is really more educational than other varieties is unclear; many users swear that the quality of the trip depends on the strain of mushroom involved, with some being more visual, others more physical, and so on, while others insist a cube is a cube is a cube (potency does vary). Either way, P. cubensis is known for producing a variety of enjoyable “symptoms,” including euphoria, introspection, and hallucinations—and some that are less-pleasant, such as nausea and sometimes  anxiety. Serious side effects are rare but possible. There is some evidence that psilocybin, the psychoactive substance present in Psilocybe mushrooms, may have some medicinal value, though it is still extremely illegal in most jurisdictions (one reason there hasn’t been much research).