If you're wanting to add PHP to GeneratePress without touching any core files, you have a few options.
1. Create a plugin
Sounds scary? It's not!
First, install this plugin: Pluginception
Once installed and activated, go to "Plugins > Create a New Plugin" and fill out your options.
Once you've named and created your plugin, you'll be directed to a file which you can add your custom PHP to.
If you don't want to do all of the above, we created one for you. Download Simple PHP.
To install, save the above .zip to your computer, and go to "Plugins > Add New > Upload Plugin" in your Dashboard.
Once activated, go to "Appearance > Simple PHP" and you'll be directed to a file where you can add your custom PHP.
2. Use a plugin
Of course, there's plugins out there that allow you to add your functions using their interface. This can be a good option, but then you're kind of relying on them to keep it up to date with WordPress. Here's one option I've found: Code Snippets