In their quest for amrit (the elixir for spiritual immortality), the demons and the gods collaborated to churn the milky ocean. Mount Mandara was selected as the churning stick while Vasuki, the king of serpents, was the churning rope. Vishnu himself took the form of a tortoise and dived into the ocean to support with his back the base of the mountain. After Vasuki had been wrapped around the mountain, the demons laid hold of one end of the rope and the gods the other. They churned the ocean for a thousand years.
But the first thing to rise from the murky depths of the ocean was Kalakut, a deadly poison. This had to be disposed of before the churning could proceed. Lord Shiva, sitting aloof at a distance, was approached. He shook himself out of his deep meditation and surveyed the scene. Then he swallowed the poison in one gulp, and his throat promptly turned blue (earning him the moniker Nilkantha, or Blue Throat). Shiva needed to cool off the immense heat generated by Kalakut, so he dived into Gosainkunda Lake. In the memory of this selfless act by Lord Shiva, pilgrims take a holy dip in the lake and wash away their sins. But just like for the great Shiva, this pilgrimage is not risk free.
Starting from Dhunche in Rasuwa district, it is best to take four to five nights to reach the lake so you are properly acclimatised. You need to listen to your body and not push ahead relentlessly, disregarding the symptoms of acute mountain sickness (AMS), chiefly headache and nausea. If you don't have a sulpha allergy, diamox will help prevent and treat AMS.
You should also drink about two litres of clean water (boiled or treated with chlorine/iodine tablets) per day to avoid dehydration. Proper rain gear and a supply of table salt to deal with annoying leech bites will come in handy. The lake area is completely packed during Janai Purnima so being psychologically prepared for some of the hardships will help. But you will be amply rewarded for your efforts.