All gameplay modes GE:S are coded in Python as outlined on the Python wiki page. Making your own custom gameplay is rather easy as we have set up most of the ground work already.
To make a new gameplay mode all you have to do is create a new python file in the python/ges/GamePlay folder (make sure it ends with '.py'). Inside your python file you must make a new class that inherits from the base gameplay (GEScenario) or another gameplay. Dont forget to import the other class up the top (i.e. for GEScenario you would add 'from GamePlay import GEScenario'). Another point to remember is that your class name must be the same as the file name otherwise the dynamic loader wont be able to load your gameplay.