Assistants are an API product for developers. They are also not practical as their cost per query can be very high, maximum context length, iterative, delayed output. You want to sell a value-added service where you only find out the next day that user questions were costing you $2 a go?
GPTs are within ChatGPT plus.
You can share GPTs, and GPTs can be set to use actions to call on your own oauth domain API to provide functionality. This is like plugins: use the wolfram plugin, and it employs an API that can calculate math.
However you would bear the cost if you make an API that costs you significant money for every use, such as fulfilling general user requests with AI model calls of your own.