A gawker is a person who stares openly at someone or something. After a bad car accident on the highway, gawkers often slow down for a look.
To gawk is to gape, stare, or rubberneck without trying to hide the fact that you're doing it. A gawker is someone who does this. A gawker might eavesdrop on a neighboring table at a restaurant or peer towards a film set hoping for a glimpse of a movie star. Both gawker and gawk were coined in the US, probably from the Middle English gowen, "to stare."