thats what made it work without the runtime error for me. drop and drag the bios to the system folder. select the Reicast core and load game. thats what i was missing and i dont know where else to get the files.
i used the mupen64 core for gauntlet legends and the screen flashes and crashes on loading the first world. im just going to download gauntlet dark legacy for gamecube, its a definitive edition of legends anyhow.
if you still need help with this check out simply austin tutorial. [[you need to download his pack in the desc. then drag the extracted DC folder into system folder within your retroarch folder and done.]] dont know why it isnt in the download of retro when all the other systems just work. hope this helps