ground wire is named ground wire coz of the conenction to ground. It can be any sorta wire as long it conduct electicity and not too thick or over size as compared to other wire in your geetar.
usually for geetar work, people use multi strand wire coz its more flexible to work with. You can use single core type, but its less flexible(good thing is you can shape it anyway you like and it stay that way)
go thru those articles in the guitar nuts site. Those info in there are really useful and will help you in long run, by going it thru over, over and over again.