There are many great teachers, both today, and throughout history. Some are also scholars, some are great orators. Many wrote books. Some were also Gurus.
By one definition, and teacher is a Guru. Mother, than father, are your first Gurus. But you can have a dance Guru, a music Guru, etc. It's synonymous with teacher, and in this sense can also be a spiritual Guru.
But for me personally, the Guru is really about the Guru-devotee, or Guru-sishya relationship. I'm traditional about it, and here are a few thoughts. Not intended as an end-all, just my personal opinion.
The Guru has to be living. You can't have a personal relationship with a deceased teacher. Sri Ramana or Sri Ramakrishna were great mystics, great teachers, but one can no longer go and sit, and ask a personal question in the here and now. Living Gurus know your name. They find time for personal counsel, like an older brother, a father, and more. Almost by definition, they are NOT mass market. Becoming mass market via promotion would severely hinder the ability to find time for everyone.
Traditionally, Hindu Gurus simple do not promote themselves, or allow others to promote them. Neither are they self-declared. They come into in one of two ways ... being appointed as the head of an existing lineage by the outgoing Guru as the most qualified person to carry on the leadership, or having someone come along and beg 'Teach me'.
They may or may not enter the more public realm by writing books.
Sure there's more. maybe later. But one thing I've learned is that the concept of it, just like everything else in the umbrella called Hinduism, varies a lot from sect to sect, from person to person.