Minecraft Forge Crash

com.google.common.util.concurrent.ExecutionError: java.lang.OutOfMemoryError: GC overhead limit exceeded

Memory: 13631448 bytes (12 MB) / 254279680 bytes (242 MB) up to 254279680 bytes (242 MB)

JVM Flags: 3 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx4G -Xmx256M

your minecraft ran out of memory. and you have same 2 JVM flag. maybe you want to allocate 4G to minecraft but you wrote -Xmx twice so java ignore the first one and use the last one. because that minecraft ran out of memory because it only 256M.
so you must change the -Xmx256M to -Xmn256M or -Xmn2G if you like so.

sorry for my bad english.
~VanillaWaffle_

I don't know anything about code, but: com.google.common.util.concurrent.ExecutionError: java.lang.OutOfMemoryError: GC overhead limit exceeded

Whatever this means, seems like it may be your problem. Sorry I'm not much help.