We have taken the decision to remove the +UseG1GC options from under the -XX:+UseExperimentalVMOptions flag.
Closed as a duplicate of 6919980 which fixed the problem.
Due to a JPRT glitch, this change was undone by a subsequent merge changeset (that was actually mis-merged). I opened
6919980 : G1: remove +UseG1GC from under experimental options (second attempt)
to reapply this change.