I wanted to use groovy for a little ftp script and found this post http://www.hhhhq.org/blog/2009/05/01/ftp-using-groovy-and-ant/ Since there were several dependencies I wanted
You're right suspecting the classloader to be the root of the problem. As your code already reveals, the AntBuilder is loaded from the RootLoader, that doesn't have access to the classes loaded by the @Grab annotation. As GROOVY-3730 shows, Groovy 1.7 is going to address this problem.
However, you can solve your problem by directly using the groovy.grape.Grape.grab(Map dependency)
method, in which you can set a specific classloader that should be used to load the dependencies:
import groovy.grape.Grape;
Grape.grab(group:'ant', module:'ant', version:'1.6.5', classLoader:this.class.classLoader.rootLoader)
Grape.grab(group:'ant', module:'ant-nodeps', version:'1.6.5', classLoader:this.class.classLoader.rootLoader)
Grape.grab(group:'ant', module:'ant-apache-oro', version:'1.6.5', classLoader:this.class.classLoader.rootLoader)
Grape.grab(group:'ant', module:'ant-commons-net', version:'1.6.5', classLoader:this.class.classLoader.rootLoader)
Grape.grab(group:'commons-net', module:'commons-net', version:'1.4.1', classLoader:this.class.classLoader.rootLoader)
Grape.grab(group:'oro', module:'oro', version:'2.0.8', classLoader:this.class.classLoader.rootLoader)