Thank you for reporting. I plan to rewrite and extend the whole GC part, so I'll also make the statement clearer.
I've been using G1GC since the very day Elasticsearch supported Java 1.8 without problem. However, a few people have reported issues with it, and Elastic recommends not using it. I'd say it's up to you to evaluate the risks, but from an operational point of view, using G1GC was miraculous compared to CMS: from 10 nodes popping out of my clusters a day because of garbage collection to 1 every 2 or 3 weeks.