I'm not aware with GridFS, But if it is chunks already you can use fetch the data using GRIDFS query and use
Document class to create Node Objects. Then you can simply pass them into
VectorStoreIndex like this
documents = []
# fetch all the chunks and iterate over them to make node objects
# add other useful info like filename, any other imp info in metadata
# chunk should be of type text.
for chunks in mongo_chunks:
documents.append(Document(text=chunk, metadata={"key":"value"}))
index = VectorStoreIndex.from_documents(documents, service_context=service_context)
