ThoughtWorks Studios products use the OpenSocial gadget stack to provide UI integrations. This provides a secure, standards-based platform for displaying one application's content in another application. Part of this infrastructure is that GoCD is itself an OpenSocial gadget rendering server. In general, a gadget is simply a piece of content from another application, such as showing a Mingle card activity table inside of GoCD.
Most OpenSocial gadgets are publicly available content that do not require authentication and authorization. However, some gadgets, such as those published by the ThoughtWorks Studios products GoCD and Mingle, do require authorization. To accomplish this, GoCD's gadget rendering server supports OAuth 2.0 services.
OAuth is an open-standard that enables a user to share private protected resources (e.g. photos or financial records) which she stores on one site (or application) with another one of her commonly used sites or applications without asking her to share any passwords between the two sites. More details about OAuth can be found here.
More about Mingle gadget in GoCD can be found here.