I believe it is just GPT-4 with a custom system prompt added. You can also upload your own added knowledge base of data for the GPT to reference.

As TFA I linked to notes, "You can now create custom versions of ChatGPT that combine (1) instructions, (2) extra knowledge, and (3) any combination of skills" [numbers and italics added].

Regarding skills, "In addition to using our built-in capabilities, you can also define custom actions by making one or more APIs available to the GPT. Like plugins, actions allow GPTs to integrate external data or interact with the real-world. Connect GPTs to databases, plug them into emails, or make them your shopping assistant. For example, you could integrate a travel listings database, connect a user’s email inbox, or facilitate e-commerce orders."

For more on custom actions: https://platform.openai.com/docs/actions