2015-5-28 | 10:10
It's been nearly a month since my resources stopped loading. After many attempts I have managed to get it working.
I will now document the procedure which I believe rectified the issue:
    Add the following to build.gradle:
        idea { module { inheritOutputDirs = true } }
    Run the following gradle commands:
        gradle setupDecompWorkspace idea --refresh-dependencies
        gradle setupDevWorkspace idea --refresh-dependencies
        gradle idea
    (re)import the gradle project. (Blue arrows on gradle idea tab)

USED RESOURCES:
http://www.minecraftforge.net/forum/index.php/topic,21354.25.html