Budget brands for gears in India
Changing major features of a bike (e.g., drop/flat handlebars, gears/no gears) is rarely an economical option. There are all kinds of compatibility issues (e.g., different drop-out width, nowhere to mount a derailleur, etc.) and overcoming these takes time and money and tends to leave you with a worse bike.

Usually, the most economical and most effective way of making this kind of major change is to sell the bike you don't want and buy the bike you do want, possibly second-hand.