Using a custom maven plugin in Gradle 6.4.1


In my org we have been using Maven and have a handful of custom plugins and have decided to employ Gradle 6.4.1.

But we need to continue using the custom maven plugins.

This Old retired post, is very close to my situation - but does not work in 6.4.1
( How to download and evoke a maven plugin? (reply posted by [Hans Dockter]))

I get unable to resolve class
when used like
def mojo = new com.mycompany.maven.plugins.Alpha()

What could I be doing wrong?
Could you point me to related documentation (gradle ver 6.4.1)

thank you.