A figure of speech in the fiftieth poem of Gitanjali Song Offerings is the simile by which the beggar speaker compares the appearance of the king of kings' golden chariot in the distance to a gorgeous dream.
A simile is a figure of speech in which two dissimilar things are compared using the words like or as. In this particular case, the speaker, a humble beggar, tells us that the king of kings' golden chariot appeared "like a gorgeous dream."
The chariot, of course, isn't a dream; it's a means of transportation. But it appears to the speaker like a dream because he's sure that when the king of kings alights from his chariot, he'll give him a very generous gift of alms.
The beggar has most probably been dreaming of such a remarkable stroke of luck for a very long time, and so one can quite easily understand just why he's so excited to see the king of kings show up in his golden chariot. For the beggar, it's a dream come true.