I believe there's nothing wrong with the tutorial. In that example they haven't made any kind of availability to GC service.
However, if you want to have actual availability, first you need to decide how to determine if GC service is available or not. In that example the upper server where service belongs, availability is determined if process is available. I wouldn't do it.
Anyway what comes for the GC, it's kind of difficult to check if that is available or not since it java's internal service. Java won't do garbage collecting until it's necessary, since it quite heavy operation. Well not that heavy as it used to be.
Because you are monitoring jvm, just be happy if jvm itself is available. Don't look too closely on those availability values, sometimes other metrics will give more information. Basically availability will tell you that you have a problem. Usually it's better to monitor metric values and see those potential problems before things starts to escalate...