RPG was more oriented towards mini-computers than mainframes, from what I recall. It's niche was the IBM S/36, S/38, AS/400 and iSeries boxes. I'm sure you probably could get RPG for your S/360 or S/390 or whatever, but from what I've seen over the years, it was mostly COBOL, PL/I, and MVS Assembler on the mainframes.