Sir Gawain and the Green Knight is a very interesting poem, because it is very detailed and does present a relatively realistic, if idealised, view of a medieval landscape. The fact that he travels alone and armoured is particularly notable, but he is not the only "hero type" to be portrayed in this way. The self sufficient warrior may be a fictional notion, or it might have been possible, hard to say. As the others mention above, though, much more likely is for a knight to travel in groups, it is just safer that way for one thing.