While *IMO* it is better and easier to use Windows LoopBack if you need to run a program in conjunction with your GNS3 lab, like a network traffic analyzer, if you want or need a second "real" computer VirtualBox is your answer.
This process is relatively simple.
Fire up GNS3
Edit >> Preferences
VirtualBox >> VirtualBox
You should see a list of your VirtualBox's. Just select one and hit Finish!
Now drag over you new Virtual Box and right click >> start.
*This should open the Vbox if not open, but doesn't always work, you can open it in Vbox manually and run programs/ping from inside Vbox and Windows will figure out what route you are trying to get. Don't forget to configure the network adapter inside virtualbox! Use CMDs to make your life easier*