Abstract: To address the problems of high computational cost and low efficiency when traditional Monte Carlo methods are employed in simulations of gamma logging while drilling (LWD), which limit the application of gamma logging in real-time geosteering, a fast forward modeling method for gamma LWD was developed. Based on the operating principle of gamma logging, this method analyzed the contribution rate of the spatial position of media to the measurement response of gamma detectors and established a flux sensitivity function for gamma logging. The gamma response could be rapidly obtained by integrating the discrete values of the gamma sensitivity function with the radioactivity intensity of corresponding spatial units. Compared with traditional Monte Carlo methods, the proposed method reduced the single-point calculation time from 10 min to less than 2 ms, achieving real-time simulation calculation and significantly improving computational efficiency. With the influence of environmental factors on logging responses taken into account, the relative error between the results of this method and those of high-precision Monte Carlo simulations was within 5%, which demonstrated reliable calculation accuracy. The simulation method based on the flux sensitivity function provides a rapid simulation approach for gamma LWD in layered formations and also offers an efficient data processing technique for fast model comparison and matching in geosteering.