The Windows installer for Groovy 2.5.0-beta-2 and 2.6.0-alpha-1 are available from the usual places:
https://bintray.com/groovy/Distributions/download_file?file_path=groovy-2.5.0-beta-1-installer.exe
https://bintray.com/groovy/Distributions/download_file?file_path=groovy-2.6.0-alpha-1-installer.exe