Protein aggregation has been implicated in the pathology of several neurodegenerative diseases. A comprehensive understanding of protein aggregation is vital to the development of therapeutic strategies for neurodegenerative diseases. Recently, amyloidogenic heptapeptide GNNQQNY has emerged as a molecule of choice for fundamental studies of protein aggregation. Despite its popularity of GNNQQNY in experiments, little theoretical work has been aimed at supporting and augmenting available experimental observations. In this work we present a rigorous theoretical analysis of the aggregation pathway of amyloidogenic heptapeptide GNNQQNY. The aggregation pathway of GNNQQNY involves a rare dimerization re-attraction, which is not easily studied in conventional simulations. We have used transition path sampling Monte Carlo simulations to study the pathways involved in that dimerization. Statistical analysis of the reaction pathways shows that the dimerization reaction proceeds via a zipping mechanism, initiated with the formation of contacts at the third residue (N). We also discuss the role of the water in the dimerization pathway. Our results can also explain the structural observations of GNNQQNY aggregates that have emerged from experiments.