Utility in plone

Using plone 4.3 .i am trying examples in plone book. trying utilities examples using in instance/bin/zopepy terminal. I am using grok.GlobalUtility base class. The class is created fine but i am not able to use it by getUtility . Getting ComponentlookUpError.

The same when i use getGlobalSiteManager and register using registerUtility , it is fine. Now I am able to use by getUtility

Grok usage has been discouraged.
If you still want to learn it, try to provide a code example.
Probably your code wasn't grokked