Lex, is there an easy way to increase the default heap size when running on OSX? Perhaps just in the gradle wrapper? Admittedly, not a hard problem to solve, but you have to run it with --debug to see why it fails.
I had the same problem and was able to get around it by modifying the build/unpacked/dev.json to use 2.9.1 instead of 2.9.1-nightly... for the lwjgl libraries. I then encountered another problem that cropped up with MCP earlier this year: we need more heap space than the default gradle script allows. I specified 1024m and was able to build to completion. Hope that helps.