Gam Asn clusters with m+n=2,4,...,16 are studied using first-principles methods based on density functional theory. We demonstrate that with sufficient accuracy in numerical computation, the pseudopotential-plane-wave method is capable of predicting relative stability and composition-dependent properties of the binary clusters at the same quality level as all electron calculations. The calculated binding energy of the Gam Asn clusters decreases with increasing Ga to As ratio in the clusters and as expected, binary clusters are energetically favored compared to pure metal clusters of Ga and As formed with the same numbers of atoms. The highest occupied molecular orbital-lowest unoccupied molecular orbital gap decreases as a function of Ga concentration in the clusters, while the electron affinity and ionization energy show weak dependence on composition. The electron affinity increases slowly with Ga to As ratio, but the ionization energy shows a decreasing trend. However, there are large fluctuations in the predicted composition-dependent physical properties. {\textcopyright} 2007 The American Physical Society.