"org.openqa.selenium.WebDriverException: Unsupported Marionette protocol version 2" error coming while running my Selenium Script with Gecko Driver
"Exception in thread "main" java.lang.IllegalStateException: The path to the driver executable must be set by the webdriver.gecko.driver
Actually Selenium 3 supports to work with Mozilla Firefox using geckodriver executable just like other driver support. That's why you're getting this exception.
To resolve this issue you need to set this executable geckodriver into system property with webdriver.gecko.driver which you have already done.
Exception in thread "main" org.openqa.selenium.WebDriverException: Unsupported Marionette protocol version 2, required 3 (WARNING: The server did not provide any stacktrace information) Command duration or timeout: 5.26 seconds
Now the problem is your Mozilla version. Actually executable geckodriver supports Mozilla Firefox >= v47 that's why you're getting this exception.
To resolve this issue you need to upgrade your Mozilla Firefox >= v47 as well.