To develop a detailed picture of the microscopic structure of gelcore star polymers and to elucidate parameters of the synthetic process that might be exploited to control this structure, simulations of their synthesis were performed that were based on a particular synthetic approach. A range of results was observed from gelation at high reactant concentrations to the formation of various sizes and compositions of star polymers. Contrary to the prevailing experimental viewpoint, the simulations always suggest the production of a broad distribution of star polymer sizes. However, the GPC traces computed from simulation results are in good qualitative agreement with experiment. Topologically, the gelcore star polymers produced by simulation are not compact but, rather, sparse blobs loosely connected by filaments of linker when modeled in a good solvent. This is reflected in scaling relationships that relate polymer size (e.g., radius of gyration) and degree of polymerization. The arm-core composition is observed to be stoichiometric, strongly reflecting relative reactant concentrations during the synthesis. Reactions within star polymers that result in greater intramolecular cross-linking compete with those between star polymers that result in the production of larger star polymers from the joining of smaller ones. The balance in this competition can be controlled through the overall reactant concentration to limit and control resulting star polymer size. Therefore, the mean size, as well as the mean number of arms, can be controlled during synthesis by careful tuning of the overall ratio of the arm and linker reactant concentrations and the total reactant concentration.