First, mark it as "Question".
I forked and copied this repository. And I succeed to setupDecompWorkspace , build with idea. But I didn't create a gradle.properties file. Then, unluckily, when I chose the application "Minecraft Client" and run, something wrong happens. I am also unavailable to use gradle build to get Railcraft.jar . When I have free time I will provide more information.

gradle.properties is required

But failed. The same reason. See:
https://gist.github.com/liach/47281ff972d65564c042
I added gradle.properties file.
It seems that the gradle.properties was not used.

That looks like the same error that older versions of Forge threw with Java 8. And it does appear that you are using a Java 8 JVM.

Succeed after using Java 7. Close now. Thanks.