It's GCC 4.9.1 + binutils, modified to target 68k and classic MacOS from OS X or Linux. It includes a resource compiler and other tools so you can write System 6 apps in C++11 (and more importantly, gain the speed of compiling on a modern system instead of a real or emulated 680x0 Mac).
Setup is kinda rough, but it does work if you follow the instructions.
Yeah, I eventually got that setup and working, although it took multiple tries on OSX for me. I played around with it a bit, but I'm still a bit wary of the somewhat convoluted process of assembling the resulting app, mainly because I haven't fully wrapped my head around it yet.
I've been messing with mpw a bit as another way to do classic mac development on modern systems. It doesn't have the modern features and native compilation speed advantages of retro68, but I also have a little more confidence and familiarity in it running the original MPW tools in emulation.