GelDosimetry allows you to automatically compute the curve on many samples and also evaluates the error and how it is distributed in the image (error may be different in different regions of the phantom). These additional features may also mean more complexity - most importantly, you need a ground truth image of the phantom, which can come from a calibration CT scanner or you can construct it manually.
I would recommend to use a phantom with 5-10 different densities to get an idea of the shape of the intensity calibration function. That will help you to decide if line fit is sufficient or you need a polynomial fit.