The increased use of electricity in modern power systems is demanding the use of non-conventional energy sources to a large extent due to the limited stock of conventional energy sources. The present paper focuses on Gravitational Search Algorithm (GSA)-based optimal allocation and sizing of solar and wind power generating units in transmission systems. The performance of the optimization method has been tested on the IEEE-14 bus test system in terms of total generation cost minimization of conventional generating units, transmission line loss reduction, revenue earned from optimal generation of solar and wind power, and profit of non-conventional generating unit owners. The comparative study on the optimal installation of standalone solar, standalone wind, and a combined solar-wind system within the existing conventional transmission system has also been conducted to achieve better results.