The Bamboo Grails Wrapper plugin facilitates easy use of the Grails wrapper from within Atlassian Bamboo.
In order to use this plugin, you'll need an installed JDK, as well as a project that contains the Grails wrapper.
After installing the plugin, you'll have a new "Grails Wrapper" task type available to add to build plans. At a minimum, ensure that the Grails commands and Build JDK have the desired values. If desired, you can specify additional Grails options, JVM options, or an alternate working directory.
The officially recommended way to develop plugins for Atlassian applications is to install the Atlassian Plugin SDK. You can find information about it in the SDK Development documentation.
Alternatively, you should be able to build this plugin with just Maven. However, it doesn't appear that the Atlassian plugins are compatible with all versions of Maven. As of this writing, it appears to work with version 3.0.5.