The journey from Dholka to Gangtok is a long-distance trip of 2300 km. The most practical way is to take a train to New Jalpaiguri (NJP) and then hire a taxi to Gangtok. Driving is not recommended. Expect a travel time of 45+ hours by train and road.
Total Distance: 2300 km by road, 1900 km straight-line.
This is a very long journey; ensure you have enough entertainment.
October to March for the pleasant weather.