This is a new book, currently in beta, written by Mark Sobkowicz through The Pragmatic Bookshelf (https://pragprog.com/book/msgpkids/learn-game-programming-with-ruby).  I went ahead and bought it and am loving it.  The book asumes that you understand Ruby to some extent, at least the basics of building arrays, classes, and methods.  This book has helped me see the setup and goes into detail with why things are how they are.  Very good intro book for someone looking to get started with Gosu.  You will build 5 games by the end of the book and the author gives suggestions at the end of the chapters for you own personal additions to the game.

Ross