What is the average GPA score to get a PhD admission in good B-schools

What do you mean by a "good" business school?
By the way, MBA rankings should not be a criterion since research reputation is often completely different, and that's what matters (to the extent that the school matters at all) when you graduate and start looking for a job. e.g. Penn State punches way above its weight when it comes to research rankings and preparing faculty for top (MBA ranking) business schools.
Next, to your question about GPA: your GPA is important but not the only criterion. If all you have done is completed a degree, then it becomes more important. If you have done other stuff, e.g. RA'd, published research articles, etc, the GPA is a bit less important. Also, the cumulative GPA is less important than the Grade in the courses that the school cares about. If you had to submit a dissertation as part of your Master's (or even Bachelor's) degree (and do not have any publications), then you should consider submitting the dissertation with the application as a "writing sample".
No, you don't need WES. Most universities are familiar with the Indian education system and should be able to evaluate your performance from your raw transcript. By the way, the WES conversions often depend on the specific school. Also, many universities (esp. engineering schools) in India do award GPAs.