The term Gecko stands for a Web Browser engine that is inbuilt within Mozilla Firefox browser. Gecko driver acts as a proxy between Web Driver enabled clients(Eclipse, Netbeans, etc.) and Mozilla Firefox browser. In short, Gecko driver acts as a link between Selenium Web Driver tests and Mozilla Firefox browser.
Selenium uses W3C Webdriver protocol to send requests to GeckoDriver, which translates them into a protocol named Marionette. Firefox will understand the commands transmitted in the form of Marionette protocol and executes them.
System.setProperty(“webdriver.gecko.driver”, driverPath);
DesiredCapabilities capabilities = DesiredCapabilities.firefox();
capabilities.setCapability(“marionette”,true);
driver= new FirefoxDriver(capabilities);
or
FirefoxOptions options = new FirefoxOptions();
options.setLegacy(true);