Making a custom post type in GP is the same as any other theme.
First you’ll want to create your custom post type: https://codex.wordpress.org/Post_Types
You would add your PHP code into the functions.php file of your child theme.
Then you’ll want to copy a file like single.php and add it to your child theme, and rename it: single-posttypename.php
That’s the file where you can build the template handling the post type functions.
If you don’t add the template file, your post type will use the core single.php file.
Let me know if you need more info 🙂