The response of a GC will ideally only be affected by the concentration of your analyte in the sample. However, in reality this is never the case. The response will be affected by alot of external parameters that will cause the signal to drift over time. A response factor is therefore not a good way to measure purity.
In general,Rf is the ratio of the response value of the compound of interest and that of any standard compound whose identity and response value is established.
There are various methods to calculate response factor eg, (i) addition of internal standard,(ii) By external std. addition etc.