Do I have to include full gpl v2 license information in my commercial project if I use another author's work?

Question

I would like to use another author's work in my commercial project.

That author's work is licensed under GPL v2.

I'm not using full code. I'm using only part of the code. Also I modified it as per my project needs.

Now i'm confused about what kind of license information I should include.

This is the full license text.

I'm using multiple author's work in functions.php file. Some works are licensed under MIT.

1) Do I have to include full license text in my php file as a comment?

2) Do I have to create separate php file for each author's work.?