According to the 2001 census of India, the population of Gangtok "Notified Town Area" (a census unit) was 29,354. Males constituted 54% of the population and females 46%. The Gangtok subdivision of the East Sikkim district had a population of 179,376, of which 43,711 reside in urban areas of Gangtok and Tadong. Gangtok has an average literacy rate of 69.7%, higher than the national average of 59.5%: male literacy is 77.9%, and female literacy is 61.5%. In Gangtok, about 8% of the population is under 6 years of age. About 8% of Gangtok’s population live in the nine notified slums and squatter settlements, all on Government land. More people live in areas that depict slum-like characteristics but have not been notified as slums yet because they have developed on private land. Of the total urban population of Sikkim, Gangtok Notified Town Area has a share of 55.5%. Including Gangtok, East District has a share of 88% of the total urban population. The quality of life, the pace of development and availability of basic infrastructure and employment prospects has been the major cause for rapid migration to the city. With this migration, the urban services are under pressure, intensified by the lack of availability of suitable land for infrastructure development.

Ethnic Nepalis, who settled in the region during British rule, comprise the majority of Gangtok's residents. Lepchas, native to the land, and Bhutias also constitute a sizeable portion of the populace. Additionally, a large number of Tibetans have immigrated to the town. Immigrant resident communities not native to the region include the Marwaris, who own most of the shops; the Biharis, who are employed in mostly blue collar jobs and the Bengalis.

Hinduism and Buddhism are the most popular religions in Gangtok. Gangtok also has a sizeable Christian population and a small Muslim minority. The North East Presbyterian Church, Roman Catholic Church and Anjuman Mosque in Gangtok are places of worship for the religious minorities. The town has remained secular, having never witnessed any sort of inter-religious strife in its history. Nepali is the most widely spoken language in Sikkim as well as Gangtok. English and Hindi being the official language of Sikkim and India respectively, are also widely spoken and understood in most of Sikkim, particularly in Gangtok. Other languages spoken in Gangtok include Bhutia (Sikkimese), Tibetan and Lepcha.

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