Calling Groovy Scripts/Function From Java (Gradle) Project

Question

I've a Gradle Java project. Currently we're not using Groovy at all so no configuration for Groovy at all. Now I need to provide support for groovy scripts/class/function(s) which can be called from Java code.

To give you context - We need to transform some fields before writing them in a file. We want that user himself can write those transformations and need not to compile code. And we should be able to call those transformations before writing to file. For this we're thinking that we'll be writing Groovy for those transformations...

So if someone can guide me that how to achieve this step by step, Means what to add/subtract in gradle file .. add src/main/groovy package/classes... I'll really appreciate it. I tried to search on internet but could not find any help...