GPL license and passing on the rights

I am quite new to wordpress, but have been playing around with the CSS of a theme to get it to look how I want it for a friend.

I am looking at changing the name in the Credits section to my own as I have changed the apperance of the website. I have asked the author of the theme if its ok to put my name to it, which he has said is ok. (I have asked the question in this link https://wordpress.org/support/topic/credits-2?replies=2)

I haven’t changed anything in the style.css as I have created a child theme, so I assume that the copyright notices will not have been removed?

He also says in the reply that I need to inherit the GPL license and pass on the rights? Could someone explain what this means and how I can go about it? As I don’t want to be doing anything that is wrong!

Thanks in advance