Do you need to install or update Groovy? Try
java InstallGroovy groovy-binary-1.5.0.zip, this is a simple java program to download, unzip and link a Groovy binary release package into your /opt local system. The source code is also attached.
NOTE: the class file is compiled with Java 1.5, so you will need at least that to run it. I think Groovy currently support Java 1.4 though, oh well, you just can't get everything. Can you?