In project GroovyFX doing gradle build I get this error:

C:TempgroovyfxsrcmaingroovygroovyxjavafxcanvasFillArcOperation.groovy: Could not find class for Transformation Processor groovyx.javafx.beans.FXBindableASTTransformation declared by groovyx.javafx.beans.FXBindable

Both classes FxBindableASTTransformation and FXBindable are in the same package, in the source code.
How do I fix the classpath in Gradle, so this classes are on the classpath?