I've never really used anything other than the OpenJDK and Azuls.

How does PTC and Aicas does GC? Is it ref counted? I'm guessing they aren't doing moving collectors.

They are real time GCs, nothing to do with refcounting.

One of the founding members of Aicas is the author of "Hard Realtime Garbage Collection in Modern Object Oriented Programming Languages" book, which was done as part of his PhD.