The journey from Gangtok to Surat covers roughly 2200 km. You must first reach NJP or Bagdogra. From NJP, you can take a long-distance train to Surat Junction. Flights from Bagdogra to Surat usually connect through Delhi or Mumbai.
Total Distance: 2200 km by road, 1800 km air distance.
This is a very long journey; ensure you have a confirmed berth.
Delhi is the most convenient hub for this connection.
October to March for pleasant weather.