To get rid of the graininess, select a higher "Load Preset" from the button below in the Render Settings window.  Keep in mind, higher pixel sampling will take longer to render.  GPU means that the rendering is working off of your graphics card.  

If you did not reset the units of measure in the General Preferences, you might be using Poser Units for distance.  ATM, render without depth of field until you become more familiar with rendering.