Automatic Gain Control (AGC) is a very important component in a Global Navigation Satellite System (GNSS) receiver. Such functionality is required anytime multibit quantization is implemented so as to lower quantization losses. In GNSS receivers, where the signal power is below that of the thermal noise floor, the AGC is driven by the ambient noise environment rather than the signal power. As a result, AGC can be a valuable tool for assessing the operating environment of a GNSS receiver. AGC and its functionality are investigated throughout this paper. Behavior of this system, both in the nominal thermal noise environment as well as when the AGC circuit will be subjected to various types of interference, are examined. Its usefulness as an interference detection and estimation tool is highlighted through several tests using simulated and real signals. Moreover, the frequency assignment of the proposed GPS L5 signal is expected to face a significant interference environment as a result of other aviation systems operate in or near band. A specific AGC and analog-to-digital converter (ADC) design performing digital pulse blanking and dedicated to L5 receivers is also studied.