I am working on Google API like chat, contacts and so on...
I am stuck on developer_key as mentioned in gdata doc.
You can get this at https://code.goog
If you are only calling APIs that do not require user data, such as the Google Custom Search API, then API keys might be simpler to use than OAuth 2.0 access tokens. However, if your application already uses an OAuth 2.0 access token, then there is no need to generate an API key as well. Google ignores passed API keys if a passed OAuth 2.0 access token is already associated with the corresponding project.
Note: You must use either an OAuth 2.0 access token or an API key for all requests to Google APIs represented in the Google Developers Console. Not all APIs require authorized calls. To learn whether authorization is required for a specific call, see your API documentation.
Reference: https://developers.google.com/console/help/new/?hl=en_US#credentials-access-security-and-identity