Gelato is renderer engine plug in that uses Nvidia GPU and the 3D scene. OpenGL is code language to communicate with GPU and memory block through the script process. I am sure that Gelato is created with OpenGL, without it Gelato won't be possible because it need to know where the 3D data are placed in. They released entensions for Gelato to be able to read outside 3D formats such as Renderman RIB. If I were programmer for Gelato, I would translate outside 3D format into Nvidia card's memory block for faster rendering using OpenGL references.