> GDP itself is a mathematical model

GDP is a metric. The specific way in which we construct it is based on a small zoo of models, each working to estimate a myriad of parameters.

It's estimated via a model. We don't actually know the value of all economic activities happening in the country. We just have statisticians survey some fraction of businesses on how they're doing, and plug those result into a model to estimate how the entire country's doing.

> It's estimated via a model

So is every physical constant. The gravitational constant is a metric. We then measure it by way of models.

GDP isn’t a model per se. GDPNow is.