I want to connect a NMS to a router on GNS if it's possible, so that i can practice the "actual thing".
As Samirtow mentioned, as long as there is connectivity between NMS and Cisco, there shoudn't be any problem. Some time ago I tested some management software (sshdm) in GNS on my laptop. For some reason I didn't have connectivity between my laptop and the CISCO router in GNS, had no ping, using LAN interface. My guess it because the LAN interface was'n connected, the status of the interface was unplugged. So if that is you case, you could either connect it via any interface that is plugged-in, or create a windows loopback adapter. There is a guide here: http://www.smartpctricks.com/2014/06/connecting-gns3-real-networks.html.