Tagore was renowned for his spiritual and magical poetry. Seen as a prophet, Tagore's works functioned as lessons for one to follow in order to move people to wish to make their lives better.
Tagore's poetry, filled with beautiful language and mesmerising verse, was meant to transform the reader to a higher place- in the mind. This being said, the tone of his poetry was reflective, spiritual, and acceptance of ones desire to better them self.
Reflectively, Tagore's texts supplied readers with cultural ideals and political ideology which Tagore fully described. It was through this that Tagore was able to speak to the masses given his works were more than utterings on religion and politics.
Tagore wished to changes the world by offering his own understanding of the things around him. The tone of his writings matched this; he wanted nothing more than to offer people the ability to change their lives. Tagore's writings offered a new view of how one should live their life; something typical of those deemed a prophet.
Although I am happy to answer this question, please know that I think it is unfair to label ALL of the works of a particular author with one tone. Tone, of course, is the attitude of the author towards his or her subject. This being said, because Rabindranath Tagore's subject is almost always the same, then it's fairly safe to say that his tone is usually one of spiritual didacticism.
Rabindranath Tagore, be it through the verse of Gitanjali (Song of Offerings) or the intense plot of his "The Postmaster" or the lectures revealed in The Religion of Man, always speaks about both Eastern and Western thought, the similarities, the differences, and how to use the spirituality (especially that of the East) to make one's life better. THIS is the reason why I say that the tone is general spiritual didacticism. Because Rabindranath Tagore had a spiritual experience (some call it "Transcendental") involving unification with nature and God, he dedicated his life to the pursuit of sharing that same happiness with others. Central to his belief system is his knowledge about everything containing at least some divinity within. It is this knowledge (stemming from his visions and experiences) that led to a huge burst of creativity that lasted decades, when he produced all of his literature.
In conclusion, I want to end with one of my very favorite quotes by Rabindranath Tagore:
Every child comes with the message that God is not yet discouraged of man.
In my opinion, this sums up the spiritual didacticism tone of Rabindranath Tagore and his works as a whole. Rabindranath Tagore is teaching us about spirituality and about God's kingdom ... or at least the one "of this world."