I don’t have a set time-frame, but it’s one of the next items on my to-do list.
Here’s a good guide on child themes: http://www.hongkiat.com/blog/wordpress-child-themes-dev/
Remember, ignore the part about @import in the stylesheet – this is not necessary using GP.
What you would have to do is copy the content.php file in the generatepress folder, and paste it into your child theme’s folder.
Then alter the HTML/PHP to suit your needs.