Can a baby who soon will be 6 months old, travel with her Grandparents from US to India without her parents. She has a valid Passport and Visa and most probably they will be travelling via Virgin Atlantic from JFK (New York) to Heathrow (London) to IGI (New Delhi) (Airline is not yet confirmed). Also, what all documents they need for this travel from parents' side (like copy of their Passport, Birth Certificate of Baby, any consent letter etc.)
Yes it is possible.
You can follow the FAQ on US Customs and Border Protection for more information:
If a child (under the age of 18) is traveling with only one parent or someone who is not a parent or legal guardian, what paperwork should the adult have to indicate permission or legal authority to have that child in their care?
Due to the increasing incidents of child abductions in disputed custody cases and as possible victims of child pornography, Customs and Border Protection (CBP) strongly recommends that unless the child is accompanied by both parents, the adult have a note from the child's other parent (or, in the case of a child traveling with grandparents, uncles or aunts, sisters or brothers, friends, or in groups*, a note signed by both parents) stating "I acknowledge that my wife/husband/etc. is traveling out of the country with my son/daughter/group. He/She/They has/have my permission to do so."
Normally these need to be notarized and since no parent is traveling airline will probably request one from both parents.