On the surface this seems like a duplicate of http://jira.codehaus.org/browse/GROOVY-3851 but it's not. If the original issue was fixed, it's broken now in 1.8.6 and 2.0.5
Here's my simple example:
This example throws a ClassNotFoundException when run through the GroovyConsole, but runs just fine when you run it from the command line.
The console should read the grapesConfig.xml file on startup and use the settings there to resolve any grape references in the script. That doesn't seem to be happening.