I'm not sure. I did it because of Jenkins Pipeline DSL; I learned enough to write ~400 loc of a build script from scratch. I was able to de-sugar the DSL and wrap raw APIs with a DSL of my own design (I'd say that I "wrote a couple of helper functions", but the former sounds way cooler...). I did stumble upon some gotchas - the difference between `def` and simple assignment when the target changes, for example.
EDIT: I wonder, is that level of proficiency enough for you to at least drop the scare quotes around "learn"? I feel that putting the quotes there is rather impolite.
> did you skim some docs and just learn what Groovy should be?
As I elaborate on in the comment below, there are different levels of proficiency and I never claimed mastery - just a basic proficiency allowing me to read all of the language constructs and write, as mentioned, a simple script from scratch, with the help of the docs.
> And did you already know any Java beforehand?
Well, a bit, although I didn't work with it professionaly in the last decade. However, knowing Java wouldn't be enough to make learning Groovy that fast - I have another trump card up my sleeve when it comes to learning programming languages. You might be interested in a section of my blog here: https://klibert.pl/articles/programming_langs.html if you want to know what it is. To summarize: I simply did it more than 100 times already.