Sun glasses preferably polaroid film coated or UV protected lenses to counter sun rays and white blindness
It is advisable to night halt at Lachen for the journey from Gangtok is pretty tiring. The early morning drive to Gurudongmar is also helps tourists to acclimatise with the weather.
Take some light snacks, fruits and water and it is advisiable not to travel empty stomach. Identity proof like voter / aadhar / card / passport is a must for permits and medicines for headache, fever, sunscreen of 50 SPF or more may be carried with.
Adequate woolen clothes to keep warm as the weather is fairly chill. Caps and full sleeves clothing is desirable