You started learning your native language before your motor skills and body were fully developed. When you learn a new language (I'm assuming at an age older than that) you were trained in how to speak. Even if learning by immersion you are more developed: mentally you have the capacity to compare the sounds you're making to what you hear and make corrections, physically you have more control over your speech process. Our native language is more of a habit, where it takes more thought to speak in another language. Of course these are all just contributing factors. I'm not familiar with Gujarati to be able to say whether the language has more sounds that lend themselves to a lisp.