Just open \dev_hdd0\game\NPUB30638\USRDIR\RDR2.self with rpcs3 and it will be decrypted. Maybe you also need the .rap file in the usual place, \dev_hdd0\home\00000001\exdata
Asus N55SF, i7-2670QM (~2,8 ghz under typical load), GeForce GT 555M (only OpenGL)