Forge 1.12 + FG3 + Mixins 0.8.2

Update: Forge 1.12 + FG6 + Mixins 0.8.5

build.gradle

buildscript {
 repositories {
 maven { url = 'https://repo.spongepowered.org/repository/maven-public/' }
 }
 dependencies {
 classpath 'org.spongepowered:mixingradle:0.7-SNAPSHOT'
 }
}
apply plugin: 'org.spongepowered.mixin'
minecraft {
 runs {
 client {
 jvmArg "-Dfml.coreMods.load=com.example.examplemod.LoadingPlugin"
 }
 server {
 jvmArg "-Dfml.coreMods.load=com.example.examplemod.LoadingPlugin"
 }
 }
}
repositories {
 maven { url = 'https://repo.spongepowered.org/repository/maven-public/' }
}
dependencies {
 compile "org.spongepowered:mixin:0.8.2"
}
jar {
 from {
 configurations.compile
 .filter { "mixin-0.8.2.jar" == it.name }
 .collect { it.isDirectory() ? it : zipTree(it) }
 } {
 exclude "LICENSE.txt", "META-INF/MANIFSET.MF", "META-INF/maven/**", "META-INF/*.RSA", "META-INF/*.SF"
 }
 manifest {
 attributes([
 "FMLCorePluginContainsFMLMod": true,
 "ForceLoadAsMod" : true,
 "MixinConfigs" : "mixin.examplemod.json",
 "TweakClass" : "org.spongepowered.asm.launch.MixinTweaker",
 "TweakOrder" : 0
 ])
 }
}

resources/mixin.examplemod.json

{
 "compatibilityLevel": "JAVA_8",
 "package": "com.example.examplemod.mixin",
 "mixins": [
 "GuiMainMenuMixin"
 ]
}

com.example.examplemod.LoadingPlugin

package com.example.examplemod;
import net.minecraftforge.fml.relauncher.IFMLLoadingPlugin;
import org.spongepowered.asm.launch.MixinBootstrap;
import org.spongepowered.asm.mixin.Mixins;
import javax.annotation.Nullable;
import java.util.Map;
public class LoadingPlugin implements IFMLLoadingPlugin {
 public LoadingPlugin() {
 MixinBootstrap.init();
 Mixins.addConfiguration("mixin.examplemod.json");
 }
 @Override
 public String[] getASMTransformerClass() {
 return new String[0];
 }
 @Override
 public String getModContainerClass() {
 return null;
 }
 @Nullable
 @Override
 public String getSetupClass() {
 return null;
 }
 @Override
 public void injectData(Map<String, Object> data) {
 }
 @Override
 public String getAccessTransformerClass() {
 return null;
 }
}

com.example.examplemod.mixin.GuiMainMenuMixin

package com.example.examplemod.mixin;
import net.minecraft.client.gui.GuiMainMenu;
import org.spongepowered.asm.mixin.Mixin;
import org.spongepowered.asm.mixin.injection.At;
import org.spongepowered.asm.mixin.injection.Inject;
import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
@Mixin(GuiMainMenu.class)
public class GuiMainMenuMixin {
 @Inject(at = @At("HEAD"), method = "initGui")
 public void initGui(CallbackInfo ci) {
 System.out.println("Hello from GuiMainMenu!");
 }
}