So it turns out I had 32-bit java, which i have now updated. I can get past the :decompilemc step, but then I get stuck at another one: ':recompilemc'
* What went wrong:
Execution failed for task ':recompileMc'.
> Unable to find a javac compiler;
com.sun.tools.javac.Main is not on the classpath.
Perhaps JAVA_HOME does not point to the JDK.
It is currently set to "C:\Program Files\Java\jre1.8.0_221"