The increased use of electricity in modern power system is demanding the use of non-conventional energy sources in 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 system. The performance of optimization method has been tested on 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, profit of non-conventional generating unit owners. The comparative studies of optimal installation of only solar, only wind and both solar and wind in existing conventional transmission system have been also studied for better results.