Following this guide.
I cannot find this class. I was previously stuck on RTData missing, but I had downloaded the GameSparks UnitySDK off their Asset Store directly first, but it was so out of date I just wiped out all that information and reimported the .unitypackage straight from the site's latest. GameSparks.RT and the RTData class now exist, but RTSesison does. An IRTSession does exist, but I don't believe that is an instantiatable class and I'm not sure how the guide makes that jump. There's a missing tutorial explaining how GameSparksManager pulls Instance() and GetSession().