I'm using a key from the Developer's Console, but getting: Your private key is in an unexpected format and cannot be used. Please try again with another private key.
There are many types of keys that can be created through the Developer's Console. The one gcloud expects is one for a "Service account".
See "I'm not using Compute Engine. What do I need to do?" from the FAQ for detailed steps on how to create and obtain a JSON key.
Reference Issue: #261