JBoss 4 and 5 application servers provide two JMX MBeans Servers: the standard JMX Mbeans Server and the JBoss embedded one.
NeoLoad monitoring module (real JBoss5 or Generic JMX on JBoss5) interacts with the second MBeans server.
However, it is not Java compliant so it doesn't contain metrics relative to the JVM like the Garbage Collector.
The metrics for JBoss server can be viewed from the URL: http://localhost:8080/jmx-console/
And you will see that you do not have Garbage collection, as well. Since NeoLoad 4.0, you have the possibility to use a Generic JMX monitor that will allow you to monitor any JAVA application that uses the standard JMX Mbeans server.
You can check the Jboss documentation to know how to enable standard JMX on Jboss. When it's done, you just have to configure the JMX custom monitor in NeoLoad using the right information.
For more information, see “Monitors” section in NeoLoad documentation.