The nice thing about Gradle is that it considers Ant a first class citizen and inter-operates with it very nicely. So for all practical reasons, you can refer to the guide about using ant from gradle (link below). One of the reasons we were able to migrate to Gradle is that we used Ant in the master build script to apply functionality that used to exist in the old ant scripts (like the junitreport). You can also put a few code snippets in that document that shows how to import an ant file. The master build.gradle file itself has a few nice references on how to use ant from gradle.
So I would say if you did not make any customizations to OFBiz, then you don't need to do anything to migrate. If, on the other hand, you did make custom ant scripts let's say, then you just need to import them into your build.gradle file (whether top level or inside components).
In short, you can just write a paragraph explaining that gradle works with ant, provide the below link, and maybe a little code snippet to show how to import ant scripts. I think this would be satisfactory to users unless I'm missing something important?