The journey from Gangtok to Mangaluru covers roughly 2700 km. You must first reach NJP or Bagdogra. From NJP, you can take a long-distance train to Mangaluru Junction. Flights from Bagdogra to Mangaluru usually connect through Bangalore or Mumbai.
Total Distance: 2700 km by road, 2200 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.