This messy little demo might give you some idea about how gosu works.
Gosu engine will loop and update each object in gosu framework after they are initialized. In each iteration (20 ms), gosu called update and draw method of objects in gosu in order to update status of objects and render them as graphics on your computer.
In this small game, only WhackARuby class is in gosu. Other classes such as Player class define two functions for rendering object and update object to get involved in the game (here they are named draw). In draw function, we use built in function in gosu image class to render player (member defined in Player class). update here is not implemented cuz I move its functionality to update function in WhackARuby class.