The journey from Gangtok to Thiruvananthapuram is one of the longest in India, covering over 3000 km. You must first reach NJP or Bagdogra. From NJP, you can take a long-distance train to Thiruvananthapuram Central. Flights from Bagdogra to Thiruvananthapuram usually connect through Delhi, Mumbai, or Bangalore.
Total Distance: 3000 km by road, 2500 km air distance.
This is a very long journey; ensure you have a confirmed berth.
Bangalore is the most convenient hub for this connection.
October to March for pleasant weather.