Aims and objectives: This study examined the effect of the first language (L1) grammatical gender (GG) system on the second language (L2) gender agreement processing, particularly when L1 and L2 are asymmetric in their number of gender values such as in Spanish and German languages. Methodology: Behavioral and brain responses (ERPs) were registered while German native-speakers (16) and Spanish-German bilinguals (24) read sentences for comprehension. We manipulated gender agreement between the determiner and the noun: Der/*die Ring, “themasc/*thefem ringmasc.” The nouns were either of similar or opposite gender across languages or unique in L2 (neuter gender). Data and analysis: Behavioral responses measured participants’ ability to discriminate grammatical from ungrammatical sentences. For ERPs, N400 and P600 components were analyzed by means of a linear mixed-effect model. Findings/conclusion: Behavioral data showed that German native-speakers discriminated gender agreement violations better than Spanish-German bilinguals, irrespective of lexical gender overlap across languages. Native speakers showed a P600 effect to gender agreement violations, whereas bilinguals showed an N400-like effect to gender mismatches that involved same-gender nouns across languages and neuter gender nouns. Interestingly, opposite gender nouns between L1 and L2 that agreed in gender with the determiner were processed as incorrect for the bilinguals. These results suggest transfer effects from L2 learners’ L1 GG system that facilitate L2 gender agreement processing for similar and neuter gender nouns or hinder it for opposite gender nouns across languages. Originality: This study used ERPs to demonstrate how asymmetries in terms of the number of lexical gender categories between L1 and L2 GG systems can affect L2 learners’ ability to process L2 gender agreement in a native-like manner. Significance/implications: These findings elucidate the challenges L2 learners face when learning a three-gender value system in L2, including a great number of gender incongruences between L1 and L2 nouns and a new neuter gender category.