Hello Darek,

It is possible to use Gradle without knowing Groovy and if you keep with basic tasks you probably do not need to learn the new language.
Nevertheless, to start something more interesting or solve some non-standard issues you would benefit greatly from Groovy knowledge. Seems to me, that Gradle will push you towards Groovy in the end (which is not a bad thing btw).
But you can start without Groovy knowledge.

--
Tomek