The important messages are those that warn you of some kind of problem, and are usually in red text over green, and come with a "dong" warning sound too.
The "non important" messages are white on green, and are the informative texts like "a marshaller has been dispatched" or "deboarding requested", "waiting for you action, open exit 1" so basically everything that is not signaling any kind of problem.
Note that, even if you turn off the "waiting for your action" message by turning down the verbosity slider, GSX will STILL wait for you action, it will just no say it anymore, so the services will still not being unless you opened/closed the requested exits. To really ignore a door status, is still required to set that option for each door in the airplane configuration.