We have recently proposed [9], the use of 'fast-light' media to obtain ultrahigh precision rotation sensing capabilities. The scheme relies on producing a critically anomalous dispersion (CAD), in a suitable dispersive medium, which is introduced in the arms of a Sagnac interferometer. We present here an experimental investigation of the anomalous dispersion properties of bi-frequency Raman gain in Rb vapor, with the goal of using this medium for producing the CAD condition. A heterodyne phase measurement technique is used to measure accurately the index variation associated with the dispersion. The slope of the negative linear dispersion (or group index) is experimentally varied by more than two orders of magnitude while changing the frequency separation between pump fields, responsible for producing gain. Using this result, we have identified the experimental