It's trying to get an old version of that Codec... just force a newer one by adding an explicit annotation. I also updated the selenium dependency versions to run against my latest n greatest FireFox, v 29.0 at the time of this writing:
@Grapes([
@Grab("org.gebish:geb-core:0.9.2"),
@Grab(group='commons-codec', module='commons-codec', version='1.9'),
@Grab("org.seleniumhq.selenium:selenium-firefox-driver:2.41.0"),
@Grab("org.seleniumhq.selenium:selenium-support:2.41.0")
])
And of course make sure you have FireFox installed on your machine.