Studies of gender in various L2s have proven this area to be particularly difficult to master, even for highly proficient L2 speakers (Franceschina, 2005; McCarthy, 2009; Grüter et al., 2012, inter alia). Although disagreement exists as to why this might be, L2 learners characteristically: (1) display low accuracy levels for gender agreement and/or assignment compared to L1 speakers; (2) adopt a default form, typically the masculine form overextending to feminine contexts in dual gender systems like Spanish. Two views vie to explain non-target-likeness, representational versus computational. Representational accounts such as the failed functional feature hypothesis (FFFH), for which age of onset and L1 transfer play a deterministic role on the acquisition of uninterpretable features, compete with computational accounts such as the missing surface inflection hypothesis (MSIH), for which difficulty in retrieval of appropriate spell-out forms under task pressure accounts for non-target-likeness and is inconsequential to features' underlying representation. Heritage research, however, allows researchers to test age of onset in the acquisition of gender given heritage language speakers are often raised bilingually from very early ages. Despite results from heritage studies showing a remarkable convergence to L2 speakers (Rodina et al. 2020; Bianchi, 2012; Polinsky, 2008), the FFFH and MSIH have not yet been sufficiently considered with HL speakers. The present chapter reports a study aimed at further testing the plausibility of the FFFH and MSIH for heritage language populations and better explaining variability in knowledge of grammatical gender in heritage grammars. The ultimate attainment of Italian gender was scrutinized by comparing highly-proficient Swedish-dominant L2 speakers to highly-proficient Swedish-dominant heritage speakers of Italian and L1 speakers of Italian in Italy. An oral structural priming task and a timed GJT elicited production, judgment, and response time data on knowledge of masculine and feminine gender assignment between nouns and agreeing accusative clitics. Consistent with previous L2 and heritage research, results show the heritage and L2 speakers are more alike than the monolinguals in terms of accuracy, suggesting age of onset is not implicated in ultimate attainment as posited by the FFFH. Additionally, the heritage group was significantly faster in responding correctly to feminine items, suggesting masculine forms to be particularly vulnerable during the processing of gender. This finding is consistent with processing accounts such as the MSIH which posit masculine forms to act as defaults. I argue that, despite most results being consistent with the MSIH and the presence of gender in the L1 playing a facilitative role in ultimate attainment of Italian gender, recourse to grammar external factors need complement current theories proposed to explain non-target-likeness in heritage language knowledge. © 2023 Walter de Gruyter GmbH, Berlin/Boston. All rights reserved.