Hi there,
GP doesn’t have a built-in way to implement custom schema – this is usually handled by an SEO plugin as you’ve mentioned.
To be very honest, the pro version of Rankmath is probably your best option if you aren’t familiar with implementing schema as it’s a bit tricky from what I understand – especially if it needs to be dynamic.
Otherwise, you can try contacting Codeable suggested at the bottom of this page to see if they can help you to implement a manual solution:
https://generatepress.com/what-support-includes/
Then you might want to consider removing the schema from GP as well:
https://docs.generatepress.com/article/generate_schema_type/