Hi,
Currently the Announce plugin uses Growl on OSX to display notifications. In my experience, this did not work (I tried to install it via homebrew) and given that recent versions are distributed with iTunes (and not free) I would not say it is a developer friendly tool (hard to install IMHO).
Recent versions of OXS comes with the notification center that could be used with the Announce plugin. It is fairly easy to use via command line (osascript -e ‘display notification “Lorem ipsum dolor sit amet” with title “Title”’) and does not require additional installation as far as I know. There are one caveat though, clicking on the notification opens the script editor (http://apple.stackexchange.com/questions/57412/how-can-i-trigger-a-notification-center-notification-from-an-applescript-or-shel), terminal-notifier (https://github.com/julienXX/terminal-notifier) could be used as well (available via homebrew).
Implementing an Announcer (https://github.com/gradle/gradle/tree/master/subprojects/announce/src/main/java/org/gradle/api/plugins/announce/internal) using osascript seems fairly easy and I was wondering if you would consider a pull request for this.