This is an example of personification, where the sun is given human qualities; like a person gathering a warm coat around their shoulders.

See this definition of the word gather: http://www.writersevents.com/Words_Starting_with_G/garter_snake_gaucho/gather_oneself_together_definition.html In this case, the writer is using definition 11. Think about how you feel in the morning when you wake up. Your consciousness may feel "spread out" (the opposite of gathered together) because your mind may be thinking about your recent dream, while your body is in your bed. Also, the things you need to begin your day are spread over your house (e.g. your toothbrush is in the bathroom, your clothes are in the closet). When you "gather yourself", you get organized and ready for your daily activities by gathering your mind, gathering your toothbrush, gathering your clothes, until you are organized and ready. In this passage, the author is using a technique called personification. As you know, the sun is not really a person who can "gather itself" like you do in the morning. Instead, the author is using that phrase to give the impression that the sun had to get ready for its journey across the sky. When I read this passage, I think the character from the passage perceived dawn as a slow and gentle process due to this phrase.