This paper demonstrates a simulation-assisted measurement technique to determine the gain of an antenna accurately in an open test site. This technique is based on two-antenna gain determination method using Friis equation, with the actual measurement test setup re-created in CST Studio Suite for simulation. A log-periodic dipole antenna is used to validate the gain determination technique in this paper. The simulation of two-antenna gain determination method is demonstrated in this paper to estimate an appropriate separation distance between the two antennas that needs to be used for actual measurements. Estimating and determining the separation distance using several simulations saves time as it eliminates the need to perform the measurement at various separation distances. The measured realized gain obtained using this technique provides good agreement with the simulation and thus validates the accuracy of this technique. The proposed gain determination technique is a cost-effective technique as it requires the use of open test site, which serves as an alternative to expensive measurements performed in anechoic chambers.