The gas-assisted gravity drainage (GAGD) process has been suggested to improve oil recovery in both secondary and tertiary stages through immiscible and miscible injection modes. In contrast to continuous gas injection and water-alternating gas, the GAGD process takes advantage of the natural segregation of reservoir fluids to provide gravity-stable oil displacement and improve oil recovery. In the GAGD process, the gas is injected through vertical wells at the top of the reservoir to formulate a gas cap that allows oil and water to drain downward to the reservoir bottom, where horizontal producer(s) are placed. Extensive experimental works and limited reservoir-scale evaluation studies have been conducted to test the effectiveness of the GAGD process performance. In this paper, a comprehensive literature review is presented to summarize all of the references about concepts, principles, and field-scale evaluations of the GAGD process. Particularly, this paper presents an introduction to the mechanisms of CO2-rock-fluid interactions, gas enhanced oil recovery injection approaches, the GAGD process physical model, the factors influencing the GAGD process, and a review of all of the previous field-scale evaluation studies. Furthermore, the validation of the GAGD process in reservoir-scale applications is fully discussed by focusing on its weaknesses with respect to the optimal implementation design for achieving maximum oil recovery.