This paper uses a variant of the IMF's Global Economy Model (GEM) to estimate the macroeconomic benefits of Yemen's accession into the Gulf Cooperation Council (GCC). After calibrating the model to Yemen and the GCC block, several simulations are carried out to estimate the potential impact of economic integration on both regions. The paper draws two fundamental conclusions. First, regional integration enhances competition which produces large economic benefits for both Yemen and the GCC. In particular, we show that in some cases economic integration can increase GDP in Yemen by up to 14 percent and in the GCC by up to 7 percent over the long run. Second, even if market structures do not improve substantially, GCC enlargement can still generate substantial spillover gains in each block. More specifically, one measure of economic prosperity measured by consumption can increase by up to 7 percent in Yemen and up to 8 percent in the GCC.