Developer
Hello Sharron you can add this in your template files, follow this:
1- Inside the Generatepress folder (wordpress/wp-content/themes/generatepress) you will find a template called: “content-single.php” copy that file.
2- Now go to your child theme folder (wordpress/wp-content/themes/your-child-theme) and paste that file.
3- Now with an code/text editor open that file.
4- Inside that file search for that line of code (11 line):
<h1 class="entry-title" itemprop="headline"><?php the_title(); ?></h1>
5- Replace for this:
<h1 class="entry-title" itemprop="headline"><?php the_title(); ?></h1>
<h2><?php the_subtitle(); ?></h2>Save the file and test.
*If your child theme already have this template file, jump to step 3.