The TextMate bundle is available from Github
The complete instructions on how to install bundles from MacroMates can be found here.
The abbreviated instructions are:
1) Download the .zip file or .gtz file from Github.
2) Note that a file was downloaded that looks something like this: alkemist-groovy.tmbundle-040db42.tar.gz
2) Double click it to extract the files.
3) Note that a directory was created that looks something like this: alkemist-groovy.tmbundle-040db42
4) Rename the directory to alkemist-groovy.tmbundle
5) When prompted with a warning about changing the extension, accept it as okay.
6) Note that the icon for the directory changes from a file icon to a tmbundle icon.
7) Double click the tmbundle icon.
8) Note that a tmbundle named "Groovy" has been installed in TextMate.
For help using TextMate for Groovy development, consider joining the Google Group.