Custom GPTs are plugins, just more streamlined. It's still just a carefully written system prompt + some basic middleware scanning the output and occasionally taking over.

The UX may be different, sure, but there's no technical difference and no technical innovation here. The main value of both plugins and custom "GPTs"[0] is that they're first party. You can build or buy better implementations, but it won't be the "GPTs".

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[0] - Kudos for whoever at OpenAI that approved calling those "GPTs", for selecting a term that maximizes confusion not just about their offering, but screws with people's comprehension of LLMs in general.