There are two possible causes of lumpiness or graininess in a gelatin cheesecake. 1. The cream cheese: The cream cheese was not room temperature when you started mixing. Either that, or you did not blend it till it was completely smooth. If you don't want to wait a few hours for your cream cheese to get to room temperature, most microwaves have an auto defrost function that is a lifesaver! 2. The gelatin: You can reduce the natural lumpiness of gelatin by processing the gelatin/water mixture in a blender and then adding it to the bowl with the rest of the filling ingredients and mix it in thoroughly. Part of the reason the gelatin does this is because it is bloomed in so little water. You can also try using more water to bloom the gelatin, but be aware that your finished cheesecake will be softer.