An update: our account manager that the GPT models are available to accounts with a committed spend agreement, but not to the paygo customers. We are a paygo customer, we have not reached the threshold of committed spend yet. I think most of the smaller accounts in Azure Databricks are the same.
As a workaround, we set up the GPT models in Azure AI Foundry, and then configure a serving endpoint in Mosaic AI Gateway to capture traces. This works fine for us, and actually AI Foundry's prompt and response filtering is much better than Mosaic's is, so we come out a little ahead.
So, if you don't see the GPT 5.x models in the Models list, or see them but are told "this account is not provisioned", it's probably because you are paygo and you need to spend more and sign an agreement to unlock those models.