I found the cause of an error with the slots GUI

error: Override is not a repeatable annotation type @Override
 

I Found way to fix it:

1. (In MCreator console) click this red error dir message

2. When java file pops up you will seen doubled string "@Override"
3. Remove 1 of 2 "@Override" strings and click "Reload Gradle Project"

that should fix Gradle Files error ;)