Ha-ha, it worked. Thanks again Leo. I chose after_main_content, as you suggested. Then it reminded me to set a location in the Display Rules tab. To review for others with this issue:
1. In the WordPress Dashboard, choose: Appearance > GeneratePress > Elements > Add New
2. Choose Element Type > Hook
3. In the box, paste:
<?php if ( is_front_page() ) {
comment_form();
} ?>
4. In the Settings tab (below the box), choose after_main_content from the dropdown.
5. Check Execute PHP.
6. In the Display Rules tab, choose Location > Front Page.
7. Click Update.