The acquisition of clitics still remains a highly controversial issue in Greek acquisition literature despite the bulk of studies performed. Object clitics have been shown to be early acquired by monolingual children in terms of production rates, whereas only highly proficient bilingual children achieve target-like performance. Crucially, errors in gender marking are persistent for monolingual and bilingual children even when adult-like production rates are achieved. This study aims to readdress the acquisition of clitics in an innovative way, by entering the variable of gender in an experimental design targeting to assess production and processing by bilingual and monolingual children. Moreover, we examined the role of language proficiency (in terms of general verbal intelligence and syntactic production abilities). The groups had comparable performance in both tasks (in terms of correct responses and error distribution in production and reaction times in comprehension). However, verbal intelligence had an effect on the performance of the monolingual but not of the bilingual group in the production task, and bilingual children were overall slower in the comprehension task. Syntactic production abilities did not have any effect. We argue that gender marking affects clitic processing, and we discuss the implications of our findings for bilingual acquisition.