This study investigated the psycholinguistic and child-related variables that modulate vocabulary development and the so-called receptive-expressive gap in child L2 learners of Gaelic with English as their L1. In total, 50 6- to 8-year-old English-Gaelic bilingual children attending Gaelic-medium immersion education were administered the English and the Gaelic Crosslinguistic Receptive and Expressive Lexical Tasks (CLTs). On the Gaelic CLT, children performed better on nouns than verbs. Accuracy was modulated by item-related variables such as the estimated age when a word is acquired and its morphophonological complexity. The receptive-expressive gap was larger in the minority L2 than in the majority L1 and did not narrow after 1 year of schooling. The gap was smaller for nouns than verbs in English but not in Gaelic. Exposure to English differentially affected the receptive-expressive gap across languages. This study offers new insights into the psycholinguistic and individual factors affecting the receptive-expressive gap in bilingual children in immersion education.