I contacted the author of Sams Teach yourself Minecraft in 24 hours. Jimmy Koene.
he was kind of enough to reply.
Below worked for me on a 2011 MBP running OS X 10.11.5
Browse to https://files.minecraftforge.net/ and click on the button in the red square.
That should give you a .zip file which you should be able to open up. I've checked it on my pc and it contains the gradlew.bat file mentioned in the book.
Kind regards,
Jimmy Koene