After a discussion with our development teams, we see that Groovy v3.0 support is not ready yet for production in Katalon Studio v7.9.0 so we will move this feature to v8.0 official release to have enough time for evaluation and testing.
Please try out v7.9.0.rc2 with Groovy 2.4.20 support and let us know if you guys have any issues with it.
We especially thank @kazurayam who spent his time to deep dive into the problem and suggest the suggestion above.
Kind Regards,