Once you have defined your JMX properties you can launch Java Mission Control, and select
File > Connect... > Create new connection. You then enter the hostname, RMI port, credentials (if enabled) and you should then be able to establish a connection to the Wowza JMX interface.
Garbage collectors can be selected via the Streaming Engine Manager (Concurrent and G1), and you can also specify your own custom settings if neither of those suffice. We generally do not recommend that these are modified unless there is a specific reason to.