If you are already familiar with Spock framework and want to carry out your UI testing with it, then you are able to make a UI testing project with WebDriver + Spock + Gradle like the example, aren’t you? Why do you bother yourself for bringing Spock into Katalon Studio? Just messy, I think.
Trying to import the jar file of Spock and related libraries into the Drivers folder of your project would be possible by using com.katalon.gradle-plugin as described in Using AWS SDK in Katalon Studio, how to resolve external dependencies with Gradle.
You would be able to find a way to run a Spock-based scripts.
But in the end you would be disappointed that you can not obtain a good report from the tests ---- all you will obtain is the output into System.out. No pretty-formated reports will be available.
As for Cucumber, Katalon Team has integrated it into the Katalon Studio. I suppose that significant portion of their efforts were made for good reporting. You would miss it for Spock in KS.