Hi Rakesh,
Clarizen view widgets can be created from most views by requesting 'Share' (click the view name and choose share on the menu). Then, you can set the options and get the URL for link/embed code. Those widgets are based on a particular view, and they cannot be created from the API.
Gantt and Roadmap widgets are related to a particular Project and they can be created from the UI or the API.
Hope this helps,
Ophir