Ans:
When travelling by train or air, the place of supply will be where the food was boarded. Also, since airlines and train services typically have a pan-India presence, both SGST and CGST are charged depending on the place of supply. In your case, the place of supply will be Bangalore, Karnataka GST, assuming you board at Bangalore and the food is also boarded at Bangalore. If your return flight is from Mumbai and the food is also boarded in Mumbai, the place of supply is Mumbai and Mumbai GST will apply. In both cases, SGST and CGST are collected.