Gecko used to be used directly by a number of consumers for implementing native applications, but Camino on OS X is the only one left. All others are long-dead

Gecko used to be used directly by a number of consumers for implementing native applications, but Camino on OS X is the only one left. All others are long-dead or switching to Webkit. Mozilla threw all of their weight behind XULRunner years ago. It probably wasn't that bad of a decision, given how much of a headache it is to use Gecko directly.