EnhancedGroovy is an IntelliJ plugin which allows Groovy annotations to add their fields to your IDE.

First, add the DSL as a compileOnly dependency to the project adding the annotations and their transformer:

repositories {
    maven { url = 'https://maven.moddinginquisition.org/releases' }
}
dependencies {
    compileOnly "com.matyrobbrt.enhancedgroovy:dsl:$dslVersion" // Each plugin version has its own dsl version with the same ID
}

Now, in order to add support for annotations, in your resources folder create a GroovyScript named enhancedgroovy/_${annotationName}.groovy, where annotationName is the name of the annotation you're adding support to.
Example: given the annotation testing.MyAnnotation, the configuration script will be in enhancedgroovy/_testing.MyAnnotation.groovy.
You can ignore the package warning, adding this at the top of your script:

//file:noinspection GrPackage

When found, the plugin will execute your script, by giving it the transformer as a com.matyrobbrt.enhancedgroovy.dsl.ClassTransformer and the annotation in question as a com.matyrobbrt.enhancedgroovy.dsl.members.Annotation. Those properties can be accessed using the this.<propertyName> syntax, and for better IDE support you may uselessly cast them to their respective types.
The example below will add a private final java.lang.String testingField field when applied to a class:

import com.matyrobbrt.enhancedgroovy.dsl.ClassTransformer

final transformer = ((ClassTransformer) this.transformer)

transformer.addField([
        'name': 'testingField',
        'type': 'java.lang.String',
        'modifiers': ['private', 'final']
])