In case you don't know, a rootkit is a virus specialized in hiding itself on any system. This makes them really complicated to find and disinfect. Usually, when a rootkit infects a system it's really complicated to eliminate it, but Gmer can manage it thanks to its powerful scanner.
Once you launch Gmer, it will start to search all over your system, it will then show you a list with everything that you have loaded in the memory so that you can decide if there's anything suspicious. Thus, if Gmer has left a virus behind without deactivating it, you can do so manually.
Gmer is capable of finding hidden processes, modifications, unclear occupied memory and a great amount of things that make rootkits undetectable. This is why it is always a good idea to run the Gmer scanner now and then to clean the system of any of these threats.