Fiber optics is the elective technology for sensing temperatures in harsh environments. Among the possible exploitable working principles, fiber Bragg gratings (FBGs) are the most widespread implementation for their excellent balance between system complexity and performance. However, despite they are well known and established, so far, metrological investigations have been limited to cases in which the sensor is used in uniform temperature conditions. This paper analyzes the response of commercial FBGs employed as temperature sensors in applications that imply large temperature gradients, such as in laser-based thermal treatments of solid tumors. A theoretical model of the sensor is implemented first and then used to evaluate its response in nonuniform temperature distributions. The model results are experimentally validated by means of a liver phantom and comparisons between different grating configurations, such as single or multiplexed with different lengths, are made.