This is about devotion to God. It could be a perfect inspirational book for religious people who believe in their callings. On the contrary, atheists like Sam Harris and Richard Dawkins might adduce that there was no divine intervention in writing these songs ; it was a matter of Tagore's motivation brought about by his past experiences. 
 I only learned from high school social studies that Rabindranath Tagore won the Nobel Prize for literature in 1913. But I never attempted to read his works which made him win one of the prestigious literary prizes in the world as long as the heart of the western writers. I was just aware of the fact that I would take a look at his pictures in awe, for he has this Jesus-like aura. For me then, I had just an impression that he was such a very enlighted literary figure. As a matter of fact,he took my fancy more when I had a nun student who recommended this book since it is her most favorite book. After getting around to it, I have come to understand why she, I guess even other religious people, like this book. 
 Gitanjali means song offerings. Tagore wanted to show his strong devotion to God by singing Him beautiful songs he drew from the deepest part of his heart. No wonder the sentences are so beautifully mesmerizing. Each line can penetrate through your heart as well as might have a profound impact upon people astray from their religious faith. 
 If you are the same with Tagore, you would opine that the God to whom Tagore is devoted is the same God you are devoted to. But if you are an avowed atheist, you could say that the God to whom Tagore is devoted could be in a pantheistic form, for the contents of Tagore's songs have something to do with all the natural environment. Therefore, offering songs do not directly and clearly refer to whoever or whatever God Tagore may have believed in. No doubt these song offerings could be subject to hermeneutic principle. 
 I may be wrong since I have not read Tagore's autobiographies nor his other works yet. Nevertheless, I was impressed by the fact that Tagore's writing styles are the product of a deep, deep impression.