Despite mentions in the OTK documentation when looking for the assertion "Retrieve OAuth 2.0 Token" in a standard Gateway/OTK install it cannot be found.
Release: Any
Component: CA API Gateway / OAuth Toolkit (OTK)
This assertion is installed as part of the Mobile API Gateway (MAG) and with MAG licensing
As a workaround create an encapsulated assertion to provide similar functionality. Attached is a sample policy provided as a guideline for doing so. The sample uses the client credentials grant type but can be modified to suit your needs.