As a developer, I want to interact with GCP via gcloud.
gcloud auth login
This obtains your credentials and stores them in ~/.config/gcloud/. Now you can run gcloud commands from your terminal and it will find your credentials automatically. Any code/SDK will not automatically pick up your creds in this case.
Reference: https://cloud.google.com/sdk/gcloud/reference/auth/login
As a developer, I want my code to interact with GCP via SDK.
gcloud auth application-default login
This obtains your credentials via a web flow and stores them in ‘the well-known location for Application Default Credentials’. Now any code/SDK you run will be able to find the credentials automatically. This is a good stand-in when you want to locally test code which would normally run on a server and use a server-side credentials file.