Hi Joachim,
Steinberg’s policy is that we are always testing new releases on the last two OS versions. GA4 was released in August 2014 before the release of 10.10. Hence it support just 10.8 and 10.9. GA4.2 has been tested on 10.9 and 10.10.
You can always give it a try and install the application on a non supported OS, but we only state that it will work properly on the supported OS.