Payment.metaClass.equals = { Payment other ->

return other != null && purpose.equals(other.getPurpose()) && inn.equals(other.getInn()) && date.equals(other.getDate()) && sum.compareTo(other.getSum()) == 0

}

//Payment.metaClass.hashCode = { ->

// log.debug('overridden hashCode called')

// 1

//}

Еще можно посмотреть

@EqualsAndHashCode(includeFields=true)