There are actually quite a lot of hard real-time, mission critical systems (mostly defense) using RTSJ (real-time specification for Java) implementations in production, but I don't know how many make use of realtime GC (RTSJ allows for a semi-manual memory management, much simpler than malloc/free but not as easy as a full GC). Some RTSJ implementations have a realtime GC, like IBM's WebSphere Real Time (http://www-03.ibm.com/software/products/us/en/real-time/) -- that's the one using Metronome -- and Aicas Jamaica VM (http://www.aicas.com/). Sun/Oracle also had an RTSJ JVM with a realtime GC (said to be better than Metronome), but it seems to have been discontinued recently.
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There are actually quite a lot of hard real-time, mission critical systems (mostly defense) using RTSJ (real-time specification for Java) implementations in production, but I don't know how many make use of realtime GC (RTSJ allows for a semi-manual memory management, much simpler than malloc/free but not as easy as a full GC). Some RTSJ implementations have a realtime GC, like IBM's WebSphere Real Time (http://www-03.ibm.com/software/products/us/en/real-time/) -- that's the one using Metronome -- and Aicas Jamaica VM (http://www.aicas.com/). Sun/Oracle also had an RTSJ JVM with a realtime GC (said to be better than Metronome), but it seems to have been discontinued recently.