Update 2014-01-24: At least one other user has verified that it now works, so please make sure you try again if you are still having problems.
Update 2014-01-23: I've recently checked in some bugfixes to the libgdx gradle support which has been merged into the daily builds. Assuming you use the daylies they should appear within 24 hours. At my end the new bugfixes makes "./gradlew ios:assemble launchIPhoneSimulator" work out of the box after creating a new project with the GdxSetup class.
Original response below:
First, make sure you use at least version 0.0.2 of the gradle-robovm-plugin. Secondly, try to build the sample project using the plugin from here:
https://github.com/jtakakura/robovm-sample-ios-app
The author of the plugin was kind enough to make this based on my request. I've tested the plugin with the example and it works without a hitch. It was discussed and resolved in a closed issue here:
https://github.com/jtakakura/gradle-robovm-plugin/issues/3
It's also worth noting that on recent libgdx builds, the command line tool for generating libgdx application templates already uses the plugin so you should be ready to go without the need to download and/or muck with the stuff yourself (short of getting libdx and running the command line util for generating a template project).
If you already have a project that you need to adapt, look carefully at the build.gradle file in the robovm-sample-ios-app example from the author of the example. It shows you what is needed to get a robovm build going.