This study compares mastery of gender assignment and agreement in Italian by adult Italian-German bilinguals who have acquired two languages simultaneously (2L1), and by adult German highly proficient second language learners (L2ers) of Italian. Our data show that incompleteness in bilingual acquisition and in second language (L2) acquisition primarily affects gender assignment: the categorization of nouns and the interpretable GENDER feature are subject to vulnerability in the two modalities of acquisition. Overall, mastery of morpho-syntax (i.e., gender agreement) was nearly native-like for both groups of speakers, suggesting that uninterpretable features are unlikely to be subject to vulnerability in the heritage language of adult bilingual speakers and can be acquired in adult L2 acquisition. Deviances from the target in gender assignment and, to a lesser extent, in gender agreement are attributed to both language-internal (i.e., language) and language-external factors (i.e., amount of input).