Research on gender achievement gaps shows they exist, and are largest in the tails of the distribution, starting as early as Kindergarten and persisting through eighth grade. In mathematics, studies find small average gender achievement gaps and larger systematically male-favoring gaps among the highest achieving students. This paper seeks to understand underlying patterns in how gender achievement gaps grow, shrink or stay constant as students move through elementary school and middle school. The focus of this paper is not only to capture these trends for average gender achievement gaps, but also for gaps in the tails of the achievement distribution as those have been demonstrated critical areas of analysis by prior work in the field and may exhibit different patterns than those at the mean. Understanding when gaps change, and in what direction, provides important information about the mechanisms that may produce the changes the gaps and highlight critical time-points for intervention, sparking future lines of research in this field. This study focuses on gender achievement gaps within the United States, with particular attention to how these gaps change as students progress from first through eighth grades. It is a longitudinal study of changes in the gaps over grades, terms and cohorts. The gaps are estimated at the district level, providing a higher resolution analysis of patterns of change. The population of study is U.S. school districts serving first through eighth grade students that use the Northwest Evaluation Association (NWEA) Measures of Academic Progress (MAP) assessment. To enable comparability across grades, this study uses metric-free measures of the mean and tail gender achievement gap, specifically the V-Statistic and the Proportion-Adjusted Relative Difference. This study finds that gender achievement gaps in both mathematics and reading change meaningfully as students progress through grades. Specifically, changes in the gaps are best captured using a fully nonparametric trend on grade-by-term. Figures and a table are appended.