That's GC duration time but the article indicates:

"This is because Zing uses a unique collector called C4 (Continuously Concurrent Compacting Collector) that allows pauseless garbage collection regardless of the Java heap size (up to 8 Terabytes)."

Azul's GC runs concurrrently with the app so those GC durations occur without a pause. If you think about it, there's no reason why GC requires a pause in order to work, with the right conditions, GC should be able to analyze and remove dead allocations without stopping an app.