The usual cause of a 500 Internal Server error with Gravity PDF is a lack of WP Memory. So first, try increasing this to 256MB or higher and see if that resolves your issue.
The next most common reason is a problem with the installation or upgrade of Gravity PDF. Go to the Plugins section in WordPress admin area, deactivate and delete Gravity PDF, and then follow the plugin installation steps to reinstall Gravity PDF.
If the issue isn't resolved, you need to do some debugging. The WP Debugging plugin by Andy Fragen is an easy way to enable debug mode in WordPress. All you need to do is install and activate the plugin. Once activated, view the PDF via the admin area and check for any errors.
If no errors appear when debug mode activated, it could be masked by another plugin. You can check by installing the Health Check & Troubleshooting plugin and enable troubleshooting mode for both Gravity Forms and Gravity PDF. Gravity Forms has an excellent guide on how to troubleshoot using this plugin.
If you aren't sure how to fix the error yourself get in touch with our friendly support team.