Gmail contextual gadget - What is the updated way to create?

[Aharon via StackOverflow]

I want to develop an in-house contextual gadget for gmail.
The problem is that Google changed the way to do that. It began on November 2013, and in October 2014 the old way entirely depracated.

So:

• Which web page should I follow?
  
• Which Google service should I use?
• Does in-house contextual gadget still exist, or maybe it has been depracated?

Examples:

• The Google Apps Engine (GAE) in code.google.com/googleapps/console doesn't let me deploy the application since it requires OAuth 2.0. Am I supposed to use developers console (in console.developers.google.com)?
• There is a web page https://developers.google.com/gmail/contextual_gadgets. Is it updated?

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http://stackoverflow.com/questions/26203249/gmail-contextual-gadget-what-is-the-updated-way-to-create