I am not using GSAS software. But the agreement R factors that you want to know might be in the output file of your refinement. I think, there is no need of calculation to find those values. (I might be wrong as I am not using this software.)
Are you looking for formulas? My recommendation is to read the manual. Try this: https://subversion.xray.aps.anl.gov/EXPGUI/gsas/all/GSAS%20Manual.pdf Please see pages 167-168 and around.