Where does gcloud store its defaults?

Following the Quick Start at

https://cloud.google.com/kubernetes-engine/docs/quickstart

I set a default project like this

$ gcloud config set project lfs258

Updated property [core/project].

$ gcloud config get-value project

lfs258

The project lfs258 doesn't exist on my GCP account, though, so I'm surprised that I could set it as the default.

Where does gcloud store its defaults?  kubectl stores them in .kube/config but I can't find a similar config file for gcloud.

Thansk.