The G1 gc was introduced as release somewhere in JDK1.6 (AFAIR update18).
But as Java1.6 is EOL I would recommend to use Java7 with AS7 EAP6.
I've used the G1 strategy for test, but not in a real production environment.
Not strictly answering the question. If you want the reference to say that G1GC is supported it is here http://www.oracle.com/technetwork/java/javase/7u4-relnotes-1575007.html but that is JDK7 update 4.
I trawled the JDK6 release notes here http://www.oracle.com/technetwork/java/javase/releasenotes-136954.html but didn't see anything saying it is now supported on 6, but I may have missed it as there is a lot.