We study the gain variations in the HRC-I over the duration of the Chandra mission. We analyze calibration observations of AR Lac obtained yearly at the nominal aimpoint and at 20 offset locations on the detector. We show that the gain is declining, and that the time dependence of the gain can be modeled generally as a linear decrease in PHAs. We describe the spatial and temporal characteristics of the gain decline and discuss the creation of time-dependent gain correction maps. These maps are used to convert PHAs to PI channels, thereby removing spatial and temporal dependence, and allowing source pulse-height distributions to be compared directly regardless of observation date or location on the detector.