That means that they use a special case called nominative as marker of the subject both in transitive and intransitive sentences. None of the other cases shows gender markers. If you are interested in further questions concerning case and gender look at the page referring to nouns . Nevertheless it has often been maintained that an early stage of Proto-Indoeuropean was an ergative language, although there is no Indoeuropean language of this type. Well-known ergative languages are the Basque and the Georgian languages for example.
Ergative languages resemble the passive voice construction of nominative languages.
The same subject case is used in intransitive sentences and marks the agent there. I do not want to prove this claim but I will try to build a model of how the hypothetic ergativic stage of early Proto-Indoeuropean could have worked. First of all, it is necessary to say that the current nominative must either be a new case or a reinterpreted oblique one. With the nominative in its current function you see simply a nominative and not an ergative language. The case of the patient in the transitive sentence has not been changed anyway.