There is no tool with user interface for that. We have two extensions for dosimetry: Gel Dosimetry Analysis and Film Dosimetry Analysis. They can take yo through the whole process, but even those cannot calculate gamma between two PDDs. However, it is an extremely simple calculation, which you can even do in Excel. You need to decide how much of your workflow you want to do in Slicer. If it’s way more than this comparison, then we can write a simple script that can do this. But since your data is in Excel, which Slicer cannot produce, it is probably mostly in Matlab + Excel.
If you give us more information, then we can help, but at this point I can only do guesswork.