Hi there,
the simplest method is to use Hooks.
Either action hooks that you can access via code or the Hook Element to inject your custom field data:
https://docs.generatepress.com/?s=hook+element
Or Filter hooks that can be used to ‘change or add to’ existing theme elements such as the post meta. Which would allow you add custom field date inline to say the Date Meta or Author byline.
How will you be adding the custom fields ? Maybe the Advanced Custom Fields plugin.
Happy to help with either method.