How other browser uses gecko.
All the organizations and companies other then Mozilla uses the gecko so easily just because it is small, lightweight, and open source. Many programmers do have wish that they can add web browsing feature to their application but do not want to develop the whole software on their own. For adding the browsing feature to any software can use gecko as a back bone of their browsing functionality.
Gecko is different from web browser.
Basically Gecko is providing the foundation to display content on the screen which includes layout engine and a complementary set of browser components. However this all the components are not packaged by the gecko along with other interface modules in user friendly application such as Mozilla Firefox.